Multimaterial thermally drawn piezoelectric fibers

ABSTRACT

Disclosed are fibers that include a composite of at least three different materials, where the at least three different materials include a conductor, an insulator, and a non-centrosymmetric material, and where each material is disposed in one or more different cross-sectional regions of the fiber, with each region extending along a common length of the fiber.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application No.61/363,152, filed on Jul. 9, 2010, the entire contents of which areincorporated herein by reference.

STATEMENT AS TO FEDERALLY SPONSORED RESEARCH

This invention was made with government support under Grant No.DMR0213282 awarded by the National Science Foundation and under GrantNo. W911NF-07-D-0004 and Contract No. DAAD19-03-1-0357 awarded by theArmy Research Office. The government has certain rights in theinvention.

TECHNICAL FIELD

This disclosure relates to fibers and other structures that includematerials which convert electric signals to acoustic signals, andconvert acoustic signals to electric signals.

BACKGROUND

Fibers are ubiquitous materials that appear in a range of applicationsfrom natural and polymeric textile fabrics used in clothes to fibersmade of silica glass that carry optical signals and can be fabricated ina variety of lengths, from a few centimeters to hundreds of kilometers.Typically, fibers are made of a single material, e.g., silica glass.Recently, new classes of fibers have emerged which combine amultiplicity of different materials. The different materials arearranged in specific geometries to enable functions such as transmissionor reflection of light, detection of light, and thermal detection.

SUMMARY

In a first aspect, the disclosure features fibers that include acomposite of at least three different materials, where the at leastthree different materials include a conductor, an insulator, and anon-centrosymmetric material, and where each material is disposed in oneor more different cross-sectional regions of the fiber, with each regionextending along a common length of the fiber.

Embodiments of the fibers can include any one or more of the followingfeatures.

At a common fiber draw temperature, each of the at least three differentmaterials can have a viscosity that is less than about 10⁷ Poise. At thecommon fiber draw temperature, each of the at least three differentmaterials can maintain structural integrity. At the common fiber drawtemperature, each of the at least three different materials can maintainits chemical composition.

The non-centrosymmetric material can include a crystalline material. Thenon-centrosymmetric material can include a piezoelectric material. Thepiezoelectric material can include a ferroelectric material.

The conductor can be a first conductor, and the fiber can include asecond conductor, the second conductor being disposed in one or moreadditional cross-sectional regions of the fiber that extend along thecommon length of the fiber. The first conductor can have a viscositythat is greater than about 10² Poise at the common fiber drawtemperature to enable large surface area contact with thenon-centrosymmetric material. The second conductor can electricallycontact the first conductor along the common length of the fiber. Thesecond conductor can have a conductivity greater than that of the firstconductor to improve conductivity along the extended length of thefiber.

A first set of the cross-sectional regions can include the first andsecond conductors in electrical contact with one another define a firstelectrode, a second set of the cross-sectional regions can include thefirst and second conductors in electrical contact with one anotherdefine a second electrode, one of the cross-sectional regions caninclude the non-centrosymmetric material to define an active region, andthe first and second electrodes can be positioned on opposite sides ofthe active region. The first conductor in each of the electrodes cancontact the non-centrosymmetric material in the active region.

The fiber can include a spacer material positioned between thenon-centrosymmetric material in the active region and the firstconductor in each of the electrodes. The spacer material can be athermoplastic material.

A cross-sectional shape of the conductor can have a maximum lengthmeasured along an outer surface of the conductor and a thicknessmeasured in a direction orthogonal to the outer surface, where a ratioof the maximum length to the thickness is 3 or more. The fiber can havea length of 10 centimeters or more, and a maximum cross-sectionaldimension of 2 mm or less.

The fiber can have an outer perimeter that is one of circular,elliptical, rectangular, square, triangular, and polygonal in shape. Thefirst conductor in each of the electrodes can extend to an outer surfaceof the fiber along at least a portion of the common length of the fiber.

The fiber can be drawn from a preform having a length that is smallerthan a length of the fiber. The at least three different materials canbe disposed in an all-solid fiber cross section. The non-centrosymmetricmaterial can sustain a field of 3 MV/m or more when an electricalpotential difference is applied between the first and second electrodes.

The non-centrosymmetric material in the active region can cause anacoustic waveform to be emitted from the fiber when an electricalpotential difference is applied between the first and second electrodes.A frequency of the acoustic waveform can correspond to a frequency ofthe electrical potential difference.

The non-centrosymmetric material in the active region can generate anelectrical potential difference corresponding to an electrical waveformbetween the first and second electrodes when an acoustic waveformimpinges on the non-centrosymmetric material. A frequency of theelectrical waveform can correspond to a frequency of the acousticwaveform.

The fiber can include one or more additional regions positioned withinthe cross-section of the fiber and including materials with differentoptical properties, the one or more additional regions forming anoptical transmission element extending along at least a portion of thecommon length of the fiber. At the common fiber draw temperature, eachof the materials within the one or more additional regions can have aviscosity that is less than about 10⁷ Poise and can maintain structuralintegrity and chemical composition.

The fiber can include one or more additional regions positioned withinthe cross-section of the fiber and including materials with differentoptical properties, the one or more additional regions forming anoptical device extending along at least a portion of the common lengthof the fiber. At the common fiber draw temperature, each of thematerials within the one or more additional regions can have a viscositythat is less than about 10⁷ Poise and can maintain structural integrityand chemical composition.

The fiber can include one or more additional regions positioned withinthe cross-section of the fiber and including materials with differentelectrical properties, the one or more additional regions forming anelectronic device extending along at least a portion of the commonlength of the fiber. At the common fiber draw temperature, each of thematerials within the one or more additional regions can have a viscositythat is less than about 10⁷ Poise and can maintain structural integrityand chemical composition.

The insulator can include a polymeric insulating material. The polymericinsulating material can include a material having a high glasstransition temperature. The polymeric insulating material can include atleast one of a polyimide material, a polysulfone material, apolycarbonate material, a polymethacrylate material, a polyestermaterial, a polyacrylate material, a polyether sulfone material, acyclic olefin material, and a fluorinated polymer material. Thepolymeric insulating material can include a thermoplastic material. Theinsulator can include a high temperature insulating material. The hightemperature insulating material can include silica and/or silica glass.

The first conductor can include a composite of a host material andconducting particulates. The host material can include at least onematerial selected from the group consisting of a polycarbonate material,a polyethylene material, an acrylonitrile-butadiene-styrene copolymermaterial, an acetal copolymer material, a polypropylene material, apolyvinylidene fluoride material, and a polyetherimide material. Theconducting particulates can include at least one material selected fromthe group consisting of carbon particles, carbon fibers, carbonnanotubes, and stainless steel fibers. The first conductor can be moreviscous than the second conductor to enable large surface area contactwith the non-centrosymmetric material.

The second conductor can include one or more metals. The secondconductor can include an alloy of two or more metals. The one or moremetals can include at least one of bismuth, lead, tin, indium, cadmium,gallium, copper, aluminum, silver, gold, and zinc. The second conductorcan be more conductive than the first conductor to decrease resistivityalong the length of the fiber relative to a fiber having only the firstconductor.

The non-centrosymmetric material can include at least one of apoly(vinylidene fluoride) material, a copolymer material of vinylidenefluoride and trifluoroethylene, a polyvinyl chloride material, acopolymer material of vinyl acetate and vinylidene cyanide, a nylonpolymer material, a nylon copolymer material, and a polyacrylonitrilematerial. The non-centrosymmetric material can include a hightemperature ceramic material such as lead zirconate titanate, quartz,barium titanate, and cadmium sulfide.

An electrical resistivity of the first and/or second conductor can be10⁵ ohm·m or less. An electrical resistivity of the insulator can be 10⁸ohm·m or more. A dielectric strength of the insulator can be 10 MV/m ormore.

The fiber can include a reservoir positioned within the fiber. Thereservoir can be positioned at least partially within a region thatincludes the insulator or at least partially within a region thatincludes the first conductor. The non-centrosymmetric material can format least one wall of the reservoir.

The non-centrosymmetric material can cause an acoustic waveform to beemitted from the fiber when an electrical potential difference isapplied between the first and second electrodes. The acoustic waveformcan be configured to cause a substance present within the reservoir toleave the reservoir through a valve positioned in a wall of thereservoir. The valve can be positioned so that the substance leaving thereservoir emerges from an end of the fiber. The valve can be positionedso that the substance leaving the reservoir emerges into a hollow regionof the fiber. The valve can include a material having a permeability forthe substance that can be varied by introducing an acoustic waveforminto the material. The substance can include at least one of: (a) aplurality of particles having a particle size that can be varied byintroducing an acoustic waveform into the substance; (b) a viscositythat can be varied by introducing an acoustic waveform into thesubstance; and (c) a diffusivity that can be varied by introducing anacoustic waveform into the substance.

A drug delivery system can include any of the fibers disclosed herein,and an electrical source coupled to the fiber(s) to selectively causedelivery of a drug material in the reservoir.

An acoustic wave detector can include any of the fibers disclosedherein, and an electrical detector coupled to the fiber(s) to detect anelectrical signal produced by the non-centrosymmetric material inresponse to the acoustic wave. The detector can include a plurality offibers, wherein the plurality of fibers can be configured to form afiber array.

An acoustic wave generator can include any of the fibers disclosedherein, and an electrical source coupled to the fiber(s) to selectivelycause the non-centrosymmetric material in the fiber(s) to generate theacoustic wave.

Embodiments of the fibers can also include any of the other features oraspects disclosed herein, as appropriate.

In another aspect, the disclosure features methods for producing fibersthat include a composite of at least three different materials, themethods including: (a) assembling a fiber preform that includes the atleast three different materials, where the at least three differentmaterials include a conductor, an insulator, and a non-centrosymmetricmaterial; and (b) drawing the preform at a fiber draw temperature into afiber that includes each of the at least three different materialsdisposed in one or more different cross-sectional regions of the fiber.

Embodiments of the methods can include any one or more of the followingfeatures.

At the fiber draw temperature, each of the at least three differentmaterials can have a viscosity that is less than about 10⁷ Poise. Eachof the at least three different materials can maintain structuralintegrity and chemical composition when the preform is drawn into thefiber.

The method can include maintaining a drawing tension of 5 grams or moreper square millimeter of the preform cross-sectional area as the preformis drawn to form the fiber. The method can include cooling the fiberafter the preform is drawn, where the non-centrosymmetric materialsolidifies in a non-centrosymmetric solid phase when the fiber iscooled. The method can include annealing the fiber for a period of onehour or more at a temperature of between 120° and 150° C.

A cross-sectional region that includes the non-centrosymmetric materialcan define an active region, and the conductor can be disposed in afirst set of cross-sectional regions that define a first electrode, andin a second set of cross-sectional regions that define a secondelectrode, the first and second electrodes being positioned on oppositesides of the active region.

The method can include applying an electrical potential differencebetween the first and second electrodes to align domains within thenon-centrosymmetric material. Applying the electrical potentialdifference can include applying a direct current potential difference of10 V or more per micrometer of thickness of the non-centrosymmetricmaterial.

The relative positions and cross-sectional dimensions of each of the atleast three different materials can be substantially the same in thepreform and the fiber. The fiber can have a length that is at leastabout 100 times greater than a length of the preform. The fiber can havea maximum cross-sectional dimension that is at least about 10 timessmaller than a maximum cross-sectional dimension of the preform.

The method can include consolidating the preform prior to drawing thepreform into a fiber. Consolidating the preform can include heating thepreform under vacuum.

The fiber draw temperature can be greater than a melting temperature ora glass transition temperature of the conductor. The fiber drawtemperature can be between about 120° C. and about 500° C.

The preform can have a length of less than 50 cm. The preform can bedrawn to form a fiber having a length of 1 meter or more.

The insulator can include a polymeric insulating material. The polymericinsulating material can include at least one of a polyimide material, apolysulfone material, a polycarbonate material, a polymethacrylatematerial, a polyester material, a polyacrylate material, a polyethersulfone material, a cyclic olefin material, and a fluorinated polymermaterial. The insulator can include a high temperature insulatingmaterial. The high temperature insulating material can include silicaand/or silica glass.

The conductor can include a composite of a host material and conductingparticulates. The host material can include at least one materialselected from the group consisting of a polycarbonate material, apolyethylene material, an acrylonitrile-butadiene-styrene copolymermaterial, an acetal copolymer material, a polypropylene material, apolyvinylidene fluoride material, and a polyetherimide material. Theconducting particulates can include at least one material selected fromthe group consisting of carbon particles, carbon fibers, carbonnanotubes, and stainless steel fibers.

The conductor can be a first conductor, and the at least three differentmaterials can include a second conductor disposed in one or moreadditional cross-sectional regions of the fiber. The second conductorcan be disposed in the first and second sets of cross-sectional regions.The second conductor can include one or more metals. The one or moremetals can include at least one of bismuth, lead, tin, indium, cadmium,gallium, copper, aluminum, silver, gold, and zinc. The first conductorcan have a viscosity that is greater than about 10² Poise at the fiberdraw temperature. Specifically, the first conductor can be more viscousthan the second conductor to enable large surface area contact with thenon-centrosymmetric material, while the second conductor can be moreconductive than the first conductor to decrease resistivity along thelength of the fiber relative to a fiber having only the first conductor.

The second conductor can electrically contact the first conductor in thefirst and second sets of cross-sectional regions. The first conductor ineach of the first and second sets of cross-sectional regions can contactthe non-centrosymmetric material in the active region.

The at least three materials can include a spacer material positionedbetween the non-centrosymmetric material in the active region and thefirst conductor in each of the electrodes. The spacer material caninclude a thermoplastic material.

The non-centrosymmetric material can include a crystalline material. Thenon-centrosymmetric material can include at least one of apoly(vinylidene fluoride) material, a copolymer material of vinylidenefluoride and trifluoroethylene, a polyvinyl chloride material, acopolymer material of vinyl acetate and vinylidene cyanide, a nylonpolymer material, a nylon copolymer material, and a polyacrylonitrilematerial. The non-centrosymmetric material can include a hightemperature ceramic material such as lead zirconate titanate, quartz,barium titanate, and cadmium sulfide.

The method can include applying an oxidation inhibitor to the secondconductor prior to drawing the preform into a fiber. The method caninclude applying a wetting promoter to the second conductor prior todrawing the preform into a fiber. The method can include applying a fluxto the second conductor prior to drawing the preform into a fiber.

Assembling the fiber preform can include wrapping a layer of one of thematerials around another one of the materials. Assembling the fiberpreform can include evaporating a layer of one of the materials or asemiconducting material onto another one of the materials. Assemblingthe fiber preform can include at least one of: (a) evaporating one ormore layers of the conductor onto the non-centrosymmetric material; and(b) wrapping one or more layers of the conductor around thenon-centrosymmetric material. Assembling the fiber preform can includeat least one of: (a) evaporating a layer of the insulator onto theconductor; and (b) wrapping a layer of the insulator around theconductor.

The conductor can be a first conductor and the at least three materialscan include a second conductor, and assembling the fiber preform caninclude applying the second conductor to a portion of a surface of thefirst conductor. Applying the second conductor to a portion of thesurface of the first conductor can include applying a liquid polymersolution to the portion of the surface of the first conductor, andpositioning the second conductor to contact at least a portion of theliquid polymer solution on the first conductor. The first and secondconductors can extend substantially along the entire length of the drawnfiber.

Assembling the fiber preform can include positioning a sacrificialpreform element within the preform to define a hollow fiber region, andremoving the sacrificial preform element prior to drawing the preform.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. All publications, patentapplications, patents, and other references mentioned herein areincorporated by reference in their entirety. In case of conflict, thepresent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and notintended to be limiting.

The details of one or more embodiments are set forth in the accompanyingdrawings and description. Other features and advantages will be apparentfrom the description, drawings, and claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is a cross-sectional view of a fiber.

FIG. 1B is a perspective view of the fiber shown in FIG. 1A.

FIG. 2 is a flow chart showing a series of steps in a fiber fabricationprocess.

FIG. 3 is a cross-sectional view of a fiber with an ellipticalcross-sectional shape.

FIG. 4 is a cross-sectional view of a fiber with a triangularcross-sectional shape.

FIG. 5 is a cross-sectional view of a fiber with a squarecross-sectional shape.

FIG. 6 is a cross-sectional view of a fiber with a rectangularcross-sectional shape.

FIG. 7 is a cross-sectional view of a fiber with a symmetricalcross-sectional shape.

FIG. 8 is a cross-sectional view of a fiber with a non-centrosymmetricmaterial in an offset position relative to a central axis of the fiber.

FIG. 9 is a cross-sectional view of a fiber with more than onenon-centrosymmetric material.

FIG. 10 is a cross-sectional view of a fiber with a non-centrosymmetricmaterial that has symmetry different from the symmetry of the fibersurfaces.

FIG. 11 is a cross-sectional view of a fiber with electrodes positionedin grooves formed in surfaces of conductive materials.

FIG. 12A is a cross-sectional view of a fiber that includes a planarmaterial stack.

FIG. 12B is a perspective view of the fiber of FIG. 12A.

FIG. 13 is a cross-sectional view of a fiber with a folded internalstructure.

FIG. 14 is a perspective view of a fiber connected to an externalcontrol circuit.

FIG. 15A is a perspective view of a fiber with recesses formed in itsinsulating material.

FIG. 15B is a cross-sectional view of the fiber of FIG. 15A.

FIG. 16 is a sectional view of a fiber with a reservoir.

FIG. 17A is a scanning electron microscope image of a cross-sectionalsurface of a cylindrical fiber.

FIG. 17B is a scanning electron microscope image of a cross-sectionalsurface of a fiber that includes a planar material stack.

FIG. 17C is a scanning electron microscope image of a cross-sectionalsurface of a fiber with a folded internal geometry.

FIG. 18 is a schematic diagram of an acoustic transmission testapparatus.

FIG. 19A is a plot showing a comparison between measured and referenceacoustic signals for a fiber used as an acoustic detector.

FIG. 19B is a plot showing a comparison between measured and referenceacoustic signals for a fiber used as an acoustic transmitter.

FIG. 20 is a schematic diagram of a cylindrical fiber preform.

FIG. 21 is a schematic diagram of an optical vibrometer apparatus.

FIG. 22A is a schematic diagram of an apparatus used to measure acousticresponses of drawn fibers.

FIG. 22B is a series of plots showing response amplitude as a functionof time delay for a piezoelectric fiber.

FIG. 22C is a plot of acoustic signal as a function of frequency for aflat rectangular piezoelectric fiber.

FIG. 23 is a schematic diagram of a rectangular fiber preform.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Fibers have been produced in a wide range of geometries andconfigurations, and can include many different materials. For example,U.S. Pat. No. 7,295,734 discloses fibers that include conducting,insulating, and semiconducting materials. The fibers are fabricated frompreforms and drawn out to extended lengths so that the diameter of thedrawn fiber can be smaller than the diameter of the preform by a factorof 10 or more. Methods for fabricating such fibers are disclosed, forexample, in U.S. Pat. No. 7,295,734, and in U.S. Patent ApplicationPublication No. US 2008/0087047, the entire contents of each of whichare incorporated herein by reference.

However, typical fibers are not configured to either electricallygenerate or electrically detect acoustic waveforms, in part because suchfibers are not able to convert acoustic waveforms to electrical signalsand vice versa. The materials which are present in conventional fiberscan include materials with varying degrees of electrical conductivity,indices of refraction, and other physical, optical, and electronicproperties. In general, such materials are not capable ofinterconverting between acoustic and electrical waveforms.

A potential route to fabricating fibers with acoustic transductioncapability (e.g., fibers that can convert acoustic signals to electricalsignals, and vice versa) would be to introduce piezoelectric materialsinto the fibers. Embedded piezoelectric domains or regions would allowfibers to be electrically actuated over a broad range of frequencies,and would also allow fibers to function as sensitive acoustic detectors.Unfortunately, for the most part fibers have thus far been made ofmaterials in disordered, glassy states, precluding the crystallinesymmetry that is required for piezoelectricity.

Just as there are many benefits arising from the ability to propagateoptical signals along extended lengths of optical fiber, it would alsobe beneficial to be able to generate and/or detect acoustic waveforms infibers by electrically contacting such fibers. The present disclosurefeatures multimaterial piezoelectric fibers and fabrication methods forconstructing fibers with engineered acoustic properties. The fiberspermit generation of acoustic signals along their lengths and therebyfunction as extended electrical-acoustic transducers. The fibers alsopermit detection of acoustic signals present in the fibers by convertingthe acoustic signals to electrical signals. The embedded piezoelectricelement(s) can be integrated with other multimaterial fiber elements,including other acoustic elements and/or optical elements such asFabry-Perot cavities and photonic bandgap structures. These combinationsof fibers and other elements can be used for a variety of applications,including tunable dispersion optical transmission devices, fabrics thatcan rapidly modulate incoming optical signals, and forming fabric arraysfor large-area detection and/or transmission of acoustic waves.

In some embodiments, the fibers disclosed herein include one or morepiezoelectric materials. The piezoelectric effect is well known incertain materials and provides for the conversion of electrical energyto acoustic energy; piezoelectric devices which perform this conversionare often referred to as transducers. In a typical transducer, apiezoelectric material is positioned between two electrodes. When amechanical stress or strain is applied to the piezoelectric material,the material generates an electrical signal in the form of surfacecharge that can be measured across the two electrodes. This is referredto as the direct piezoelectric effect. Such transducers are also capableof operating according to the reverse piezoelectric effect; that is,applying a variable electric field across the electrodes produces anacoustic signal in the form of a mechanical stress or strain waveform inthe piezoelectric material.

Conventional piezoelectric transducers can be constructed from a varietyof known piezoelectric materials, including lead zirconate titanate(PZT) and poly(vinylidene fluoride). These materials can generate andpropagate acoustic signals over a broad range of frequencies from themHz region to several GHz. However, in most piezoelectric transducers,relatively large driving voltages are used to generate acoustic signalsin the piezoelectric material. Further, the brittleness of certainceramic piezoelectric materials and challenges associated withprocessing such materials have limited device geometries, length scales,and, as a result, applications.

In general, the multimaterial piezoelectric fibers disclosed herein areflexible and can be extended to long lengths by heating and drawing froma fiber preform. Moreover, driving voltages can be kept relatively lowby using high aspect ratio electrodes with large surface areas. In thismanner, acoustic signals can be readily introduced into the fibers, andcan be efficiently transmitted over long fiber lengths. The enhancedacoustic transmission properties of the fibers disclosed herein enable avariety of new applications.

Thermal drawing of a piezoelectric fiber raises a number of challenges,which span a wide range of length scales. On the hundreds of micronssize-level, the necessity to utilize crystalline materials both for thepiezoelectric layer and the electrical conductors can lead to theformation of multiple adjacent low viscosity domains of high aspectratio. These domain can undergo a significant reduction in crosssectional dimensions (during the fiber draw) and are susceptible tocapillary breakup and mixing due to flow instabilities. Also, at thetens of microns size-level, layer thickness variations either in thelateral or in the longitudinal directions can preclude the formation ofthe coercive field needed for poling. Finally on the length scale of themolecular spacing, even if capillary breakup were kinetically avertedand uniform sections of fibers at metre lengths were to emerge theymight not exhibit piezoelectricity because the stress and strainconditions necessary to induce the thermodynamic phase transition inPVDF cannot be sustained in the fiber draw process leading to anon-polar phase.

To address these challenges, certain embodiments disclosed here includea viscous and conductive polymer, such as a carbon-loadedpoly(carbonate) (CPC), that is used to confine a low viscositycrystalline piezoelectric layer during the draw process. For example,the CPC layers exhibit high viscosity (10⁵˜10⁶ Pa·s) at the drawtemperature and adequate resistivity (1˜10⁴ ohm·m) over the frequencyrange from DC to tens of MHz, thus facilitating short range (hundreds ofmicrons) charge transport on length scales associated with the fibercross section. Furthermore, more conductive metallic filaments (such asindium filaments) can be assembled within a surrounding poly(carbonate)(PC) cladding to function as a second conductor contacting the CPCviscous conductor to thereby improve conductivity over the entire lengthof the fiber. Finally, a piezoelectric polymer which crystallizes intothe appropriate phase is used. Specifically, poly(vinylidenefluoride-trifluoroethylene) copolymer (P(VDF-TrFE)) assumes theferroelectric β phase spontaneously upon solidification from the meltwithout necessitating any mechanical stress, making it particularlysuitable for the thermal fiber drawing process.

As used herein, a fiber is a structure that extends along a firstdirection (the fiber axis), and has a maximum cross-sectional dimensionextending along a second direction orthogonal to the first direction,where a length of the fiber along the first direction is larger than thefiber's maximum cross-sectional dimension by a factor of 1000 or more.

A non-centrosymmetric material is a material having a microscopicstructure that lacks inversion symmetry, as determined by x-raydiffraction, and therefore can have a macroscopic polarization.Non-centrosymmetric materials in the fibers disclosed herein can eitherbe non-centrosymmetric at the time the fiber is drawn from a preform, orthey can be centrosymmetric when drawn and later renderednon-centrosymmetric (e.g., by crystallization from a molten amorphousphase into a non-centrosymmetric crystalline solid phase).

A piezoelectric material is a material that generates an electricalsignal in response to an applied mechanical stress or strain.Piezoelectric materials also exhibit the reverse piezoelectric effect:application of a varying electric field across the material generates anacoustic signal in the form of a mechanical stress or strain in thematerial.

A conductive material is a material that has a bulk resistivity of 10⁵ohm·m or less at 20° C. An insulating material is a material that has abulk resistivity of 10⁸ ohm·m or more at 20° C.

FIG. 1A shows a cross-sectional view of an acoustic fiber 100, and FIG.1B shows a perspective view of the same fiber. In general, as shown inFIG. 1A, fiber 100 features a composite structure formed of a variety ofmaterials, each material being disposed in one or more differentcross-sectional regions of the fiber. In FIG. 1A, fiber 100 includes aregion 102 that includes a non-centrosymmetric material, regions 104 and106 that include a first conductor, and a region 108 that includes aninsulator. Fiber 100 also includes regions 110, 112, 114, and 116 thatinclude a second conductor. In fiber 100, region 102 with thenon-centrosymmetric material forms an active region. The first conductorin regions 104 and 106 directly contacts the non-centrosymmetricmaterial in region 102. The non-centrosymmetric material, the firstconductor, and the insulator are disposed within the cross-section offiber 100 to yield an all-solid fiber cross section.

Fiber 100 extends for a length L along a fiber axis. In cross-section,fiber 100 has a maximum dimension D₁ measured along a firstcross-sectional dimension, and a minimum dimension D₂ measured along asecond cross-sectional dimension. The first and second cross-sectionaldimensions are both orthogonal to the fiber axis. As shown in FIGS. 1Aand 1B, regions 102, 104, 106, 108, 110, 112, 114, and 116 (includingthe first and second conductors, the insulator, and thenon-centrosymmetric material) extend along a common length of fiber 100;this common length, in turn, extends substantially along the entirelength of fiber 100.

Fiber 100 is configured so that by applying a time-varying electricfield between regions 104 and 106, an acoustic waveform can be generatedin the active region 102 in the non-centrosymmetric material via thereverse piezoelectric effect (e.g., when the non-centrosymmetricmaterial is a piezoelectric material). Further, when thenon-centrosymmetric material is a piezoelectric material and an acousticperturbation is introduced into the material (e.g., when a mechanicalstress or strain is introduced into the fiber), the non-centrosymmetricmaterial can generate an electric field between regions 104 and 106 thatcan be detected by measuring the potential difference between theseregions. In fiber 100, a first set of regions 104, 114, and 116 forms afirst electrode in contact with the non-centrosymmetric material inactive region 102. A second set of regions 106, 110, and 112 forms asecond electrode in contact with the non-centrosymmetric material inactive region 102. The electrical potential difference generated by anacoustic waveform in the active region can be measured between the firstand second electrodes by electrically contacting the electrodes.

To efficiently generate acoustic waveforms in piezoelectricnon-centrosymmetric materials, it is important that a time-varyingelectric field is applied over a relatively large area of thepiezoelectric material. Accordingly, in fiber 100, large-aspect ratioconductors in regions 104 and 106 that extend substantially along theentire length of fiber 100 are used to apply the electric field acrossthe non-centrosymmetric material in active region 102. Further, toensure efficient electrical contact between the first conductor inregions 104 and 106, and the second conductor in regions 110, 112, 114,and 116, the second conductor in each of regions 110, 112, 114, and 116directly contacts the first conductor in either region 104 or 106 andextends substantially along the entire length of fiber 100.

The fibers disclosed herein are produced according to a thermal drawingprocess from pre-assembled fiber preforms. Preforms are typicallyconstructed by assembling multiple solid materials with differentelectrical, optical, and mechanical properties into a single macroscopicstructure. The macroscopic structure is then scaled down via a drawingprocess at elevated temperature to a diameter that is reduced by afactor of 10 or more. During this scaling down, the relative geometricalcross-sectional arrangement of materials in the fiber preform ispreserved. Thus, devices with different electrical, optical, andacoustic properties can be constructed as preforms, and then implementedin fiber form over length scales of tens of meters or more. Moreover,these fiber-based devices can be produced at low cost, and can exhibitstructural flexibility and good uniformity along their lengths.

The process of drawing long lengths of fibers from macroscopic preformsyields large area functional surfaces. Thus, for example, by drawing apreform that includes a first conductor in regions 104 and 106 into afiber, the first conductor can contact the non-centrosymmetric materialin active region 102 over a significant fraction of thenon-centrosymmetric material's entire surface area. As a result, thefirst conductor in regions 104 and 106 can be used to efficientlygenerate acoustic waveforms in the non-centrosymmetric material, and canbe used to detect acoustic waveforms in the non-centrosymmetric materialwith relatively high sensitivity. In piezoelectric fiber devices, asdiscussed above, conductors that directly contact the piezoelectricmaterial over a large surface area can generate large-amplitudemechanical responses in the piezoelectric material at relatively lowdriving voltages. By introducing such large area conductors, acousticwaveforms can be generated in the fibers disclosed herein at relativelymoderate voltages. In contrast, conventional piezoelectric transducersare typically low surface area devices due to the difficulty offabricating large-area sandwich arrays of piezoelectric materials andelectrodes. Furthermore, as the size of such conventional transducersincreases, the required driving voltage increases dramatically to theorder of several hundreds of volts for the largest transducers.

In some embodiments, the material that forms the first conductor inregions 104 and 106 is selected so that it remains sufficiently viscousat the thermal drawing temperature so that the first conductor does notundergo significant capillary break-up at the thermal drawingtemperature. By avoiding such break-up, the first conductor maintains alarge surface area contact with the non-centrosymmetric material inregion 102. In some cases, however, the high viscosity materialsselected to form the first conductor may not be as electricallyconductive as other lower viscosity materials. Accordingly, regions 110,112, 114, and 116 may each be smaller in cross-sectional area thanregions 104 and 106, and the material that forms the second conductor inthese regions can be selected on the basis of a higher conductivity thanthe material in regions 104 and 106 (e.g., the first conductor). Thesecond conductor in regions 110, 112, 114, and 116 contacts the firstconductor in regions 104 and 106 along the length of fiber 100.

By including a high conductivity material in regions 110, 112, 114, and116, the overall electrical conductivity of the entire fiber can beimproved along its entire (e.g., relative to fibers with only the firstconductor in regions 104 and 106). Through suitable selection ofmaterials, the first and second conductors together form electrodes thatprovide both a high surface area of contact with the non-centrosymmetricmaterial, and high electrical conductivity extending along the length ofthe fiber.

FIG. 2 includes a flow chart 200 showing a series of steps that can beused to fabricate the fibers disclosed herein. In the first step 202,the materials used to form a fiber are assembled into a preform. Thepreform is typically similar in both relative geometry and compositionto the final fiber, but is much larger in cross-sectional dimensions andshorter in length. Typically, for example, the preform has a length fromabout 2 to about 100 times its maximum cross-sectional dimension. Toform the fibers disclosed herein, the assembled preform includes anon-centrosymmetric material, a conductor, an insulator, and optionallya second conductor. These materials are assembled in a macroscopicstructure with a cross-sectional appearance similar to that shown inFIG. 1A.

Assembly of the various materials into the preform typically involveswrapping layers of one or more of the preform materials around otherpreform materials, and/or evaporating layers of one or more of thepreform materials onto other preform materials. Referring to FIG. 1A,for example, assembly of a preform corresponding to fiber 100 caninclude wrapping layers of the first conductor onto thenon-centrosymmetric material in active region 102 to form regions 104and 106. Alternatively, or in addition, assembly of the preform caninclude evaporating layers of the first conductor onto the interior andexterior surfaces of the non-centrosymmetric material to form regions104 and 106. Similarly, assembling the preform can include wrapping alayer of the insulator around region 106 to form region 108, and/orevaporating a layer of the insulator onto region 106 to form region 108.

One or more semiconductors can also be wrapped and/or evaporated ontomaterials during preform fabrication. Exemplary semiconductors that canbe introduced into performs include glassy semiconductors such aschalcogenide glasses.

To wrap layers of the various preform materials, the material to bewrapped is typically formed into a sheet (e.g., a polymer film) and thenrolled around the surface of the preform to add a layer to the preform.Sheets can also be rolled around individual preform materials beforethey are assembled into the preform to yield multiple functional regionswithin a single preform.

A variety of different vapor deposition techniques can also be used todeposit preform materials. Thermal evaporation, chemical vapordeposition, sputtering, and other similar techniques can be used forform layers of materials in performs. Thermal deposition, for example,can be carried out with conventional hot filament evaporation techniquesat reduced pressure (e.g., at a pressure of 10⁻⁴ Torr or less). Vacuumevaporators (e.g., Ladd Industries Model 30000) can be used in thisprocess.

Certain conductors can oxidize when exposed to high annealing and/ordrawing temperatures. Oxidized materials may not melt or flow uniformly,resulting in nonuniformities within drawn fibers. Thus, in someembodiments, one or more of various methods can be used to avoidoxidized materials in fiber preforms. In certain embodiments, forexample, an oxidation inhibitor can be used to prevent oxidation ofpreform materials. The oxidation inhibitor, which can also be a surfacewetting promoter, can be incorporated into the preform at interfacesbetween some or all of the conductors and other preform materials. Theoxidation inhibitor can be physically applied to the conductors duringpreform assembly.

In some embodiments, the oxidation inhibitor is a flux (e.g., asynthetic carboxylic acid-containing fluid or a natural rosin flux).Fluxes enhance wetting of preform materials by the conductors and helpto prevent capillary breakup of the conductors during drawing. Exemplaryfluxes that can be used include Superior No. 312 and/or Superior No. 340(both available from Superior Flux and Manufacturing Company, Cleveland,Ohio). Fluxes can be applied directly to the surfaces of conductorsand/or to surfaces of other preform materials that will contact theconductors in the assembled preform.

In certain embodiments, oxidation inhibitors can be added directly tothe conductors or to other materials in the preform. For example, anoxidation inhibitor can be added to a material that will directlycontact one or more conductors in the assembled preform. The oxidationinhibitor can naturally locate at the surface of the material to whichit is added, facilitating its action on the conductor in the preform.Alternatively, or in addition, a polymer, a semiconductor, or anothermaterial that inhibits oxidation can be included within the preform as alayer in contact with one or more conductors. Materials such as polymerscan be applied to various preform materials by wrapping and/or vapordeposition, as discussed above, and/or by other processes such as dipcoating.

In some embodiments, a fiber preform can be assembled around asacrificial preform element. Such sacrificial element can be included inthe preform to define spaces in the drawn fiber. For example, a mandrel,rod, or tube can be included in a preform to define a central hollowregion of the drawn fiber. When a sacrificial element is included in apreform, consolidation of the preform (to be discussed later) typicallyoccurs at a temperature below the glass transition temperature of thesacrificial element so that the element maintains its structuralintegrity.

In general, materials with poor surface adhesion that can withstandconsolidation can be used as sacrificial elements. Examples ofsacrificial elements that can be used in the performs disclosed hereininclude Teflon™ rods, tubes, and other structures. Teflon™ elements aretypically removed shortly after consolidation (and prior to drawing)while the preform is still hot and slightly expanded, facilitatingremoval of the sacrificial element.

In some embodiments, sacrificial elements can be removed from fiberperforms prior to drawing using other techniques. For example,sacrificial elements formed from materials such as quartz, glass, andother amorphous substances can be removed from fiber performs viachemical etching. Etching solutions that selectively target the materialof the sacrificial element (e.g., hydrofluoric acid and/or other acidbaths) can be used to remove such elements. Dry etching techniques canalso be used to remove sacrificial elements provided they canselectively attack the elements in the preform.

Next, in optional step 204, the preform can be consolidated.Consolidation can include, for example, heating and/or pressurizing thepreform so that that the assembled materials in the preform fill ininterstitial spaces within the preform and contact one another overlarger surface areas. Typically, for example, fiber preforms areconsolidated at temperatures of 100° C. or more (e.g., 125° C. or more,150° C. or more, 175° C. or more, 200° C. or more, 250° C. or more, 300°C. or more, 350° C. or more). In some embodiments, consolidation occursby heating the preform under vacuum. For example, fiber performs can beconsolidated at ambient pressures of 1 Torr or less (e.g., 0.1 Torr orless, 0.01 Torr or less, 10⁻³ Torr or less, 10⁻⁴ Torr or less, 10⁻⁵ Torror less, 10⁻⁷ Torr or less).

Then, in step 206, the preform is heated and/or pressurized and drawnout to an extended length to form a fiber in which the relativepositions and cross-sectional dimensions of each of the materials withinthe preform are substantially preserved in the drawn fiber. However,because the preform is drawn to many times its initial length, the sizesof the material features in the fiber (and the fiber diameter) are manytimes smaller than those in the corresponding preform.

Drawing of the fiber preform is typically performed at a fiber drawingtemperature of 120° C. or more (e.g., 140° C. or more, 160° C. or more,200° C. or more, 300° C. or more) and/or 500° C. or less (e.g., 480° C.or less, 460° C. or less, 440° C. or less, 420° C. or less, 400° C. orless, 350° C. or less). In general, the drawing temperature is selectedso that it exceeds the melting or glass transition temperature of thefirst conductor so that the first conductor flows when drawn.

A tension is generally applied along the length of the fiber preformduring drawing to extend the preform's length. To draw the performsdisclosed herein, draw tensions of 5 g or more per square millimeter ofthe preform's cross-sectional area (e.g., 7 g or more, 10 g or more, 15g or more, 20 g or more, 25 g or more, 30 g or more) are applied to thepreform.

An important component of the process of producing fibers according tothe flow chart in FIG. 2 is the selection of materials—includingconductors, insulators, non-centrosymmetric materials—that can beco-drawn, and can maintain both structural integrity and chemicalcomposition at the fiber draw temperature. As used herein, maintainingstructural integrity refers to a fiber material that retains itsphysical properties when it is drawn from the preform into a fiber.Thus, for example, while the material may elongate during the drawingprocess, when the material cools and solidifies in the drawn fiber, thematerial's bulk properties (e.g., brittleness, elasticity, tensilestrength, conductivity, dielectric strength, index of refraction) arethe same in the fiber as they were in the preform; the material suffersno change or degradation in its mechanical, optical, or electronicproperties as a result of the drawing process.

Further, maintaining chemical composition refers to a fiber materialthat retains its empirical chemical compositional formula during thedrawing process. During drawing, the material may change its physicalstructure—it may undergo phase transitions, for example—but theelemental composition of the material in the drawn fiber is the same asthe elemental composition of the material in the preform.

The selected materials support a matching flow rate, and do not undergoeither axial or cross-sectional capillary break-up during drawing. Toensure that suitable flow rates are achieved and break-up of fibermaterials during drawing is avoided, materials with certain viscosityproperties are selected for use. In particular, materials are selectedsuch that viscous forces within the materials oppose interfaceenergy-driven capillary break-up mechanisms, and at the same time permitthe materials to be drawn at suitable rates. Referring to FIG. 1, thematerial that forms the first conductor in regions 104 and 106 istypically a material with a sufficiently high viscosity at the fiberdrawing temperature so that capillary break-up is avoided. In thismanner, the first conductor maintains a large surface area of contactwith the non-centrosymmetric material in region 102 in the drawn fiber.In contrast, regions 110, 112, 114, and 116 can have significantlysmaller cross-sectional areas than regions 104 and 106, closer in aspectratio to the cross-sectional profile of a filament. Accordingly, thematerial that forms the second conductor in regions 110, 112, 114, and116 can be a lower viscosity—but higher conductivity—material thatimparts high electrical conductivity along the length of the drawnfiber. Because the cross-sectional area of regions 110, 112, 114, and116 is smaller, the increased tendency of the second material in theseregions to form filament-like structures upon drawing (relative to thefirst material in regions 104 and 106) does not impair the functioningof fiber 100. To the contrary, the materials in the first and secondregions of the drawn fiber together form electrodes that provide bothhigh surface areas of contact with the non-centrosymmetric material, andhigh electrical conductivity extending along the length of the fiber.

To provide stability during drawing, at least one of the materialsassembled in the preform is chosen to support the draw tension andmaintain continuous and controllable deformation during the drawingprocess. Typically, the insulator is selected to perform this function,although more generally any one or more of the materials in the preformcan function in this manner.

Further, the materials assembled in the preform (and present in thedrawn fiber) flow at a common draw temperature. While the drawtemperature can be selected based on the nature of the materials, thematerials should flow at whichever draw temperature is selected. Toensure that each of the materials flow, each material is selected suchthat it has a viscosity of 10⁷ Poise or less (e.g., 10⁶ Poise or less,10⁵ Poise or less) at the common fiber draw temperature.

Typically, the selected materials also exhibit good adhesion and/orwetting properties in the viscous and solid states. Further, thematerials are typically selected such that their thermal expansioncoefficients are relatively closely matched to prevent cracking anddelamination when the drawn fibers are cooled.

A variety of different materials can be used for each of the componentsof the preforms and fibers disclosed herein. In some embodiments, theinsulator can include a polymeric material; the polymeric material can,in certain embodiments, have a relatively high glass transitiontemperature (e.g., a glass transition temperature of 200° C. or more).An exemplary but inexhaustive list of materials that can be used asinsulators includes polyimide materials, polysulfone materials,polycarbonate materials, polymethacrylate materials, polyestermaterials, polyacrylate materials, polyether sulfone materials, cyclicolefin materials, fluorinated polymer materials, and thermoplasticmaterials. Other materials that can be used to form the insulatorinclude high temperature insulating materials such as silica and silicaglass.

Moreover, as will be discussed in greater detail later, the insulatorcan have a variety of different shapes according to the shape of thedrawn fibers. In FIG. 1A, the insulator in region 108 is in the form ofa tube (e.g., a polymeric tube). More generally, the insulator canassume other forms as well, including being shaped as a rod and as aplanar layer in certain embodiments.

Various components can also be present in the first conducting materialin regions 104 and 106. In general, the first conducting material isformed as a composite that includes a host material and a plurality ofconducting particles within the host material. Suitable host materialsinclude, for example, polycarbonate materials, polyethylene materials,acrylonitrile-butadiene-styrene copolymer materials, acetal copolymermaterials, polypropylene materials, poly(vinylidene fluoride) materials,and polyetherimide materials. Suitable conducting particles include, forexample, carbon particles, carbon fibers, carbon nanotubes, andstainless steel fibers.

The non-centrosymmetric material in active region 102 can include one ormore components selected with a view to the considerations discussedabove. In certain embodiments, the non-centrosymmetric material includesa crystalline material. In some embodiments, the non-centrosymmetricmaterial includes a piezoelectric material. Piezoelectric materials, asdiscussed above, convert mechanical stress or strain into an electricalpotential difference at the surface of the material via thepiezoelectric effect. Piezoelectric materials also exhibit the reversepiezoelectric effect in which a mechanical perturbation is induced inthe material when a time-varying electric field is applied to thematerial. In certain embodiments, the non-centrosymmetric material caninclude one or more ferroelectric materials. Ferroelectric materials, inaddition to being piezoelectric, are also pyroelectric by virtue oftheir symmetry. Further, ferroelectric materials have a macroscopicpolarization that can typically be reversed by an applied electricfield.

Exemplary non-centrosymmetric materials that can be used in the fibersdisclosed herein include polymers and copolymers. In particular, polymerand copolymer materials can include, but are not limited to,poly(vinylidene fluoride) materials, copolymers of vinylidene fluorideand trifluoroethylene, polyvinyl chloride materials, copolymers of vinylacetate and vinylidene cyanide, nylon polymer materials, nylon copolymermaterials, polyacrylonitrile materials, and a variety of hightemperature ceramic materials such as lead zirconate titanate, quartz,barium titanate, and cadmium sulfide. In some embodiments, combinationsof two or more materials can be used to form the non-centrosymmetricmaterial. For example, the non-centrosymmetric material can includepiezoelectric particulates (e.g., any of the piezoelectric materialsdisclosed herein) embedded in a matrix material (e.g., any of thematerials disclosed herein that are suitable hosts for particulates).

Additional exemplary non-centrosymmetric materials that can be used infibers include berlinite (AlPO₄); cane sugar; Rochelle salt; topaz;tourmaline-group minerals; biological materials such as bone andcollagen, tendon, silk, wood, enamel, and dentin; gallium orthophosphate(GaPO₄); langasite (La₃Ga₅SiO₁₄); ceramics with perovskite ortungsten-bronze structures; lead titanate; potassium niobate; lithiumniobate; lithium tantalate; sodium tungstate; Ba₂NaNb₅O₅; Pb₂KNb₅O₁₅;sodium potassium niobate; bismuth ferrite; and sodium niobate.

Certain materials that are used in fibers may have multiple phases, someof which are non-centrosymmetric and other which are centrosymmetric. Bycontrolling the fiber drawing conditions and/or by post-processing afterthe fiber drawing process is complete, such materials can be placed intheir non-centrosymmetric (e.g., piezoelectric and/or ferroelectricphases) in the fibers. For example, certain centrosymmetric materials,when heated to a molten phase at a certain drawing temperature, drawnunder tension, and/or cooled from the melt at particular rates, formnon-centrosymmetric crystalline phases upon solidification.

The second conductor in each of regions 110, 112, 114, and 116 typicallycontacts the first conductor in one of regions 104 and 106 to formelectrodes positioned on opposite sides of active region 102. The firstand second conductors are in contact along the common length of fiber100 (e.g., along the length over which the insulator and thenon-centrosymmetric material extend as well). As such, the secondconductor is typically formed of one or more highly conductive materialsthat are co-drawable with the other fiber materials at the common fiberdraw temperature, and maintain both structural integrity and chemicalcomposition at the draw temperature. Exemplary materials that can beused to form the second conductor include metals and alloys. Forexample, the metals and/or alloys used to form the second conductor caninclude one or more of bismuth, lead, tin, indium, cadmium, gallium,copper, aluminum, silver, gold, zinc.

During preform drawing, capillary break-up of a conductor due to flowinstabilities caused by the drawing process can lead to a fiber withfilaments of conducting material rather than a layer. Such filamentstypically have surface areas of contact with other fiber materials thatare significantly smaller than the large areas of contact afforded bythe first conducting material in fiber 100, resulting in less efficientinterconversion of electrical energy and mechanical energy in drawnfibers. Moreover, at elevated drawing temperatures, the filaments canmix with other fiber materials, resulting in inhomogeneouscross-sectional profiles (e.g., layer thickness nonuniformity in eitherthe lateral or longitudinal directions) in the drawn fibers. Thisinhomogeneity can preclude proper activation (e.g., poling) of thefibers, as discussed later.

In embodiments disclosed herein, capillary break-up during preformdrawing can be avoided by selecting the first conducting material suchthat it retains sufficiently high viscosity so that capillaries do notform during drawing. Typically, for example, to ensure that capillarybreak-up does not occur, the first conducting material has a viscosityof 10² Poise or more (e.g., 10³ Poise or more, 10⁴ Poise or more) at thecommon fiber draw temperature. Because conducting materials that aresufficiently viscous may have less than preferred conductivities,especially for carrying current over an extended length of the fiber,the second conducting material can be used. For example, the secondconducting material can be metal that contacts the first conductingmaterial along the full length of the fiber. Moreover, thecross-sectional area of this second conducting material can already besmall (e.g., like that of a filament), so that capillary break-up of thesecond conducting material during drawing is moot.

Regions 110, 112, 114, and 116 which include the second conductingmaterial can generally have a variety of different shapes. In FIG. 1A,each of these regions is implemented as an elongated strand that extendssubstantially along the length of fiber 100 such that thecross-sectional dimensions of each region are significantly smaller thanthe length of each region. In some embodiments, however, some or all ofthe regions that include the second conducting material can beimplemented as one or more layers in a planar layer stack.

In general, the fiber performs that are formed according to the methodsdisclosed herein have lengths of about 50 cm or less (e.g., 40 cm orless, 30 cm or less, 20 cm or less, 10 cm or less, 5 cm or less, 1 cm orless, 0.5 cm or less). From such a starting length, a fiber preform canbe drawn to a significantly extended length to form fiber 100. Forexample, the drawn fiber can have a length that is 100 times or more(e.g., 500 times or more, 1000 times or more, 5000 times or more, 10000times or more) longer than the length of the preform. In someembodiments, the length L of fiber 100 is 10 cm or more (e.g., 50 cm ormore, 1 meter or more, 5 meters or more, 10 meters or more, 20 meters ormore, 30 meters or more, 50 meters or more, 100 meters or more, 500meters or more, 1000 meters or more, 10000 meters or more, 50000 metersor more).

The drawn fiber can also have a maximum cross-sectional dimension thatis significantly smaller than the maximum cross-sectional dimension ofthe fiber preform. For example, in certain embodiments, the drawn fiberhas a maximum cross-sectional dimension of 2 mm or less (e.g., 1.5 mm orless, 1 mm or less, 0.5 mm or less, 0.25 mm or less, 0.1 mm or less, 50microns or less, 10 microns or less, 5 microns or less). In someembodiments, a ratio of the maximum cross-sectional dimension of thefiber preform to the maximum cross-sectional dimension of the drawnfiber is 10 or more (e.g., 10² or more, 10³ or more, 10⁴ or more). Fiber100 has a fiber axis along which its length L is measured, and a maximumcross-sectional dimension measured along a direction orthogonal to thefiber axis. In certain embodiments, a ratio of the fiber length L to thefiber's maximum cross-sectional dimension is 10³ or more (e.g., 10⁴ ormore, 10⁵ or more, 10⁶ or more, 10⁹ or more, 10¹² or more).

Following drawing of the preform to form fiber 100, each of the regionsthat include the first conductor in fiber 100 has a maximumcross-sectional length measured along an outer surface of the region anda thickness measured in a direction orthogonal to the outer surface. Forexample, referring to FIG. 1A, region 104 has a maximum cross-sectionallength that corresponds to a circumference of the outer surface ofregion 104 (e.g., the surface of the first conductor in region 104 thatcontacts the non-centrosymmetric material in active region 102), and athickness measured in a radial direction perpendicular to the outersurface of the first conductor in region 104. In some embodiments, aratio of the maximum cross-sectional length to the thickness for aregion with the first conductor is 3 or more (e.g., 5 or more, 10 ormore, 20 or more, 50 or more, 75 or more, 100 or more, 10³ or more, 10⁴or more, 10⁵ or more, 10⁶ or more). By maintaining a large ratio ofmaximum cross-sectional length to thickness, the regions with the firstconductor have a large surface area, thereby facilitating efficientpiezoelectric operation of fiber 100.

The non-centrosymmetric material, the first and second conductors, andthe insulator can also be selected according to their electricalproperties. Selecting conductors with relatively low resistivity canyield fibers that efficiently interconvert electrical and mechanicalenergy. In certain embodiments, the conductors, which are in electricalcontact with the non-centrosymmetric material, can have an electricalresistivity of 10⁵ ohm·m or less (e.g., 10⁴ ohm·m or less, 10³ ohm·m orless, 10² ohm·m or less, 10 ohm·m or less). As explained above, twodifferent conductors can be used to achieve, on the one hand, largesurface area contact with the piezoelectric material (by using a viscousconductor), and, on the other hand, high conductivity over the length ofthe fiber (by using metal filament-like regions in electrical contactwith the large surface are viscous conductor). Selecting insulators withrelatively high resistivity can help to prevent electrical discharge infibers during operation and/or activation. In some embodiments, theinsulator can have an electrical resistivity of 10⁸ ohm·m or more (e.g.,10⁹ ohm·m or more, 10¹⁰ ohm·m or more, 10¹¹ ohm·m or more, 10¹² ohm·m ormore).

Prevention of dielectric breakdown in fiber 100 can be assisted by usinglarge area conductors, which reduces the need for large driving voltagesto be applied to the non-centrosymmetric material in active region 102of fiber 100. Further, the insulator can be selected to ensure thatarcing and other dielectric breakdown events do not occur within fiber100, particularly during activation (e.g., poling) of the fiber. In someembodiments, for example, the insulator has a dielectric strength of 10V or more per micron of thickness (e.g., 15 V/μm or more, 20 V/μm ormore, 30 V/μm or more, 40 V/μm or more, 50 V/μm or more, 75 V/μm ormore, 100 V/μm or more).

In some embodiments, the non-centrosymmetric material is selected forits dielectric properties. In particular, in some embodiments, thenon-centrosymmetric material can sustain a field of 3 MV/m or more(e.g., 4 MV/m or more, 5 MV/m or more, 7 MV/m or more, 10 MV/m or more,15 MV/m or more, 20 MV/m or more, 30 MV/m or more, 50 MV/m or more) whenan electrical potential difference is applied between the firstelectrode (e.g., regions 104, 114, and 116) and the second electrode(e.g., regions 106, 110, and 112) in fiber 100.

Exemplary fibers were constructed according to the criteria discussedabove. Carbon-doped polycarbonate and polyethylene were used as thefirst conductor in the fabricated fibers, and polycarbonate andpolysulfone materials were used as the insulator. The second conductorwas formed from alloys of bismuth and tin, and from indium metal, andwas implemented in the form of metallic strands that extendedsubstantially along the entire length of the drawn fibers.

Piezoelectric poly(vinylidene fluoride) copolymers were used as thenon-centrosymmetric material. Poly(vinylidene fluoride) (PVDF) and itscopolymers are commercially available piezoelectric polymers and havehigh piezoelectricity when poled. However, piezoelectricity only occursin two phases of PVDF; moreover, direct cooling of molten PVDF typicallyyields α-PVDF, which is not a piezoelectric phase. To fabricate apiezoelectric non-centrosymmetric material, a PVDF copolymer wasprepared that consisted of poly(vinylidene fluoride-trifluoroethylene)(P(VDF-TrFE)). The P(VDF-TrFE) copolymer spontaneously solidified into apiezoelectric and ferroelectric β-phase when cooled from molten form.

Returning to the flow chart of FIG. 2, following cooling of the drawnfiber (which causes the material in active region 102 to crystallize ina non-centrosymmetric phase), in optional step 208 thenon-centrosymmetric material in the drawn fiber is activated to achievea piezoelectric response (or to enhance its existing piezoelectricity).Activation of the material typically occurs by high-field poling. Forexample, a DC voltage can be applied to the electrodes of the fiber,establishing an electric field of magnitude at least 10 V or more (e.g.,15 V or more, 20 V or more, 30 V or more, 40 V or more, 50 V or more, 75V or more, 100 V or more, 150 V or more) per μm of thickness of thenon-centrosymmetric material. The voltage can be applied for 1 minute ormore (e.g., 2 minutes or more, 3 minutes or more, 5 minutes or more, 7minutes or more, 10 minutes or more, 15 minutes or more, 20 minutes ormore, 30 minutes or more). During this period of time, the DC electricfield aligns the electric dipoles of individual domains in thenon-centrosymmetric material from an initial random or pseudo-randomorientation, achieving a macroscopic polarization in thenon-centrosymmetric material.

In some embodiments, high-field poling of the non-centrosymmetricmaterial can be preceded by a thermal annealing step to increase thecrystallinity of the non-centrosymmetric material, further increasingthe piezoelectricity of the material following poling. Thermal annealingcan occur at a temperature of 100° C. or more (e.g., 105° C. or more,110° C. or more, 115° C. or more, 120° C. or more, 125° C. or more)and/or 150° C. or less (e.g., 145° C. or less, 140° C. or less, 135° C.or less, 130° C. or less). In some embodiments, thermal annealing isperformed for a period of 1 hour or more (e.g., 1.5 hours or more, 2hours or more, 3 hours or more, 4 hours or more, 5 hours or more).

Following activation of the non-centrosymmetric material, the fiber isready for use and the process shown in flow chart 200 ends at step 210.Additional methods and systems for fabricating preforms and drawingfibers are disclosed, for example, in U.S. Pat. No. 7,295,734, and inU.S. Patent Application Publication No. US 2008/0087047.

Returning again to FIG. 1A, fiber 100 has cylindrical symmetry about thefiber axis, and the outer perimeter of fiber 100 has a circular shape.In general, however, the fibers disclosed herein can have a wide varietyof different shapes. In some embodiments, the fibers can have asymmetric cross-sectional shape as shown in FIG. 1A. Alternatively, incertain embodiments, the fibers can have an asymmetric or irregularcross-sectional shapes.

Further, in some embodiments, the fibers disclosed herein can have anouter perimeter that is symmetric but not circular. For example, theshape of the outer perimeter of the fibers can be elliptical (as shownin FIG. 3 for fiber 300), triangular (as shown in FIG. 4 for fiber 400),square (as shown in FIG. 5 for fiber 500), and rectangular (as shown inFIG. 6 for fiber 600). More generally, the shape of the outer perimeterof the fibers disclosed herein can be polygonal and can correspond to ann-sided polygon. The shape of the outer perimeter of the fibers can alsocorrespond to symmetric shapes that are not polygonal. A cross-sectionalview of an embodiment of one such fiber 700 is shown in FIG. 7. Fiberswith many different shapes can be formed by constructing suitablepreforms.

In some embodiments, non-centrosymmetric material, the first conductor,and the insulator are symmetrically oriented about the fiber axis. Moregenerally, however, some or all of these materials can be positioned inregions that are oriented asymmetrically with respect to an axis offiber 100. FIG. 8 shows a cross-sectional view of an embodiment of afiber 800 that includes an insulator positioned in a region 808. A fiberaxis 850 is oriented at the center of region 808 and extends along alength of the fiber, perpendicular to the view in FIG. 8. Fiber 800 alsoincludes a first conductor positioned in regions 804 and 806, and anon-centrosymmetric material positioned in active region 802. However,while the first conductor in regions 804 and 806 is positionedsymmetrically with respect to the non-centrosymmetric material in region802, regions 802, 804, and 806 are offset from fiber axis 850. Ingeneral, regions 802, 804, and 806 can occupy any of a variety ofdifferent offset positions relative to fiber axis 850, provided that thestructural integrity of each of the fiber materials is maintained duringdrawing.

In certain embodiments, more than one non-centrosymmetric material andcorresponding conductors can be present in a single fiber. FIG. 9 showsa cross-sectional view of an embodiment of a fiber 900 that includes aninsulator in a region 908, a non-centrosymmetric material in a region902, and a first conductor in regions 904 and 906, surrounding region902. Fiber 900 also includes the non-centrosymmetric material in aregion 922, and regions 924 and 926 which also include the firstconductor, and surround region 922. Both regions 902 and 922 in fiber900 are implemented as tube-shaped regions. The outer perimeter of eachtube of non-centrosymmetric material is a closed loop that does notcontain the center of the cross-section of fiber 900.

Although FIG. 9 shows a fiber with two regions that include thenon-centrosymmetric material and a pair of regions of the firstconductor positioned on opposite sides of each of regions 902 and 922,more generally any number of regions with one or morenon-centrosymmetric materials and/or one or more conductors can beincluded in the fibers disclosed herein, subject to geometrical andstructural material constraints during fiber drawing. The multipleregions can be symmetrically positioned with respect to fiber axis 950,as shown in FIG. 9, or asymmetrically positioned with respect to thefiber axis.

Further, in some embodiments, certain fiber elements can have shapesthat do not share the symmetry of either or both of the fiber innerperimeter and the fiber outer perimeter. For example, FIG. 10 shows afiber 1000 that includes an insulating material positioned in a region1008. The inner surface of region 1008 forms the inner surface of fiber1000, and the outer surface of region 1008 forms the outer surface offiber 1000. The cross-sectional symmetry of both the inner and outersurfaces of fiber 1000 is circular (C_(∞) about the fiber axis). Fiber1000 also includes a non-centrosymmetric material positioned in region1002, and a first conductor positioned in regions 1004 and 1006. Regions1002, 1004, and 1006 are displaced from the geometric cross-sectionalcenter of fiber 1000. Further, as shown in FIG. 10, regions 1002, 1004,and 1006 have square cross-sectional shapes with C₄ symmetry, incontrast to the circular cross-sectional shapes of the inner and outersurfaces of fiber 1000.

In general, the various regions of fiber 1000—including the regions thatinclude the non-centrosymmetric material, the first and secondconductors, and the insulator—can have a variety of differentcross-sectional shapes, including circular, triangular, square,rectangular, polygonal, and curved. The cross-sectional shapes can beregular or irregular, and symmetrical or asymmetrical. Further, as shownin FIG. 10, the cross-sectional symmetry of any of the regions in fiber1000 can be the same as, or different from, the symmetry of the innerand/or outer surfaces of fiber 1000.

Returning again to FIG. 1A, regions 110, 112, 114, and 116 are generallyused to form external electrical contacts to regions 104 and 106 and thefirst conductor therein. In FIG. 1A, the second conductor in two regions110 and 112 contacts the first conductor in region 106, and the secondconductor in two regions 114 and 116 contacts the first conductor inregion 104. Electrical signals can therefore be coupled into region 104through regions 114 and 116, and electrical signals can be coupled intoregion 106 through regions 110 and 112.

Although in FIG. 1A the first conductor in each of regions 104 and 106has two associated regions of the second conductor, more generally eachregion of the first conductor can have any number of associated regionsof the second conductor, including none at all. Further, the regions ofthe second conductor associated with each region of the first conductorcan be arranged in any geometry relative to the regions of the firstconductor, subject to geometrical and structural material constraints inthe fiber. Although regions 110, 112, 114, and 116 are depicted in FIG.1A as having rectangular cross-sectional shapes, more generally any oneor more of these regions can have any regular or irregular geometricalshape in cross-section.

As shown in FIG. 1A, regions 110, 112, 114, and 116 are positioned sothat they contact a surface of one of regions 104 and 106, therebyensuring that the first and second conductors are in contact at eachinterface. In certain embodiments, the second conductor can bepositioned in grooves formed in the first conductor or in the insulator.The grooves permit the second conductor to be stabilized in positionprior to drawing, thereby ensuring that the positions of regions of thesecond conductor remain approximately constant relative to other fiberfeatures along a length of the drawn fiber. FIG. 11 shows across-sectional view of a fiber 1100 that includes a non-centrosymmetric material in a region 1102, a first conductor in a region 1104with three associated regions 1116, 1118, and 1120 of a secondconductor, the first conductor also in a region 1106 with threeassociated electrodes 1110, 1112, and 1114, and an insulating material1108. The second conductor in regions 1116, 1118, and 1120 is positionedin grooves formed in the inner surface of region 1104. Similarly, thesecond conductor in regions 1110, 1112, and 1114 is positioned ingrooves formed in the outer surface of region 1106.

In some embodiments, the second conductor can be applied to a surface ofthe first conductor using a liquid polymer adhesive. For example,regions 1110, 1112, and 1114 of the second conductor can be applieddirectly to the surface of region 1106 of the first conductor by firstapplying a liquid polymer adhesive solution to the surface of region1106, and then positioning the second conductor so that it contacts atleast a portion of the applied liquid polymer adhesive.

In some embodiments, the regions of the non-centrosymmetric material andthe first conductor form concentric layers in the fiber. The region ofthe insulator can also form a layer that is concentric with the regionsof the non-centrosymmetric material and the first conductor. Forexample, referring again to FIG. 1A, the first conductor in regions 104and 106, the non-centrosymmetric material in region 102, the secondconductor in regions 110, 112, 114, and 116, and the insulator in region108 form concentric layers of increasing circumference moving in adirection outward from the cross-sectional center of fiber 100 towardits outer surface.

The materials can also be arranged in other geometries. FIG. 12A shows across-sectional view of a fiber 1200 with a rectangular cross-sectionalshape, and FIG. 12B shows a perspective view of the same fiber. Fiber1200 includes a layer of non-centrosymmetric material in region 1202, afirst conductor in regions 1204 and 1206, and a second conductor inregions 1212 and 1210 that contacts the first conductor in regions 1204and 1206, respectively. Region 1208 includes the insulator, and enclosesregions 1202, 1204, 1206, 1210, and 1212. Fiber 1200 extends for alength L along a fiber axis, and has cross-sectional dimensions D₁ andD₂ in directions orthogonal to the fiber axis.

In FIGS. 12A and 12B, due to the configuration and geometry of regions1202, 1204, and 1206, the non-centrosymmetric material, the firstconductor (2 layers) and the insulator form a planar layer stack. In theembodiment shown, region 1208 encloses the non-centrosymmetric materialin region 1202 and the first conductor in regions 1204 and 1206. In someembodiments, however, the insulator in region 1208 is applied to onlycertain surfaces of the planar layer stack. For example, the insulatorin region 1208 can be applied only to the upper surface of region 1204and to the lower surface of region 1206, thereby exposing side surfacesof the internal layers of the layer stack. Electrical contacts,additional insulators and/or conductors, and other materials or devicescan be applied to the exposed layer surfaces.

In certain embodiments, the cross-sectional shapes of the regions of theconductors and/or the regions of the non-centrosymmetric material can befurther chosen to improve the efficiency with which thenon-centrosymmetric material converts electrical energy to acousticwaveforms (and/or converts mechanical perturbations to electricalsignals). For example, regions of the first conductor can have across-sectional shape that features a plurality of electricallyconnected planar surfaces. When each region of the first conductor isshaped in this manner, the regions of the first conductor can bepositioned such that they form a “folded” fiber with a stack ofinterdigitated planar layers at alternating potentials.

FIG. 13 shows a cross-sectional view of an exemplary fiber 1300 thatincludes regions of a first conductor with multiple planar layers. Fiber1300 features a region 1304 that includes a first conductor with twoplanar layers 1304 a and 1304 b, and a region 1306 that also includesthe first conductor with two planar layers 1306 a and 1306 b. The twoplanar layers of the first conductor in each region are integrallyconnected to form a single continuous conductor. Regions 1304 and 1306are arranged such that the planar layers of each region form a stack inwhich planar layers from the two regions alternate, but do not touch oneanother. A non-centrosymmetric material is positioned in region 1302, inthe spaces between the planar layers of regions 1304 and 1306. Regions1310 and 1312 that include the second conductor contact regions 1306 and1304, respectively, and permit an electrical potential to be applied tothe first conductor in each of regions 1304 and 1306. Insulatingmaterial in region 1308 surrounds regions 1304 and 1306, region 1302,and regions 1310 and 1312.

Each portion of the non-centrosymmetric material in region 1302 in fiber1300 can function as an acoustic transducer. For example, planar layers1306 a and 1304 a of the first conductor in combination with the portionof the non-centrosymmetric material in region 1302 between these twolayers functions as a transducer structure. Similarly, planar layers1304 a and 1306 b of the first conductor in combination with the portionof the non-centrosymmetric material in region 1302 between these layersfunctions as another transducer. Because the layers of region 1304 aremutually electrically connected, and the layers of region 1306 are(separately) mutually electrically connected, the various portions offiber 1300 that operate as transducers do not operate independently;rather, when an electrical potential is applied between regions 1304 and1306 (e.g., via regions 1312 and 1310), acoustic waveforms can begenerated in different portions of the non-centrosymmetric material inregion 1302 at the same time.

By “folding” multiple non-centrosymmetric material regions in a singlefiber, the voltage required to drive the transducer structure formed bythe first conductor and the non-centrosymmetric material can be reducedrelative to a single-layer acoustic transmitter. In an acoustictransmitter, the amount of electrical power converted into acousticwaveforms depends not only on the volume of the piezoelectric material,but also on the magnitude of the applied electric field. To apply alarge field to a relatively small volume of material, the piezoelectricmaterial should be relatively thin and have a large surface area. Byfolding the piezoelectric material regions in a fiber as shown in FIG.13, a large-area piezoelectric material can be realized. Further, themultiple layers of the first conductor function as electrodes in FIG. 13to apply fields throughout the volume of the piezoelectric material. Asa result, both the conversion efficiency from electrical power toacoustic power, and the mechanical displacement of the piezoelectricmaterial, are increased relative to non-folded fiber geometries.

When the fiber operates as an acoustic sensor or detector, itssensitivity is enhanced relative to a single-layer acoustic detector byusing the folded geometry shown in FIG. 13. When operating as acousticdetectors, piezoelectric transducers generate surface charge in anamount proportional to the area of the piezoelectric material. Thus, by“folding” the piezoelectric material as in FIG. 13, up to hundreds oflayers of piezoelectric material can be stacked, each layer having athickness of a few microns and generating surface charges. As a result,the stacked piezoelectric material layers collectively function as ahighly sensitive detector for acoustic waveforms that enter thematerial.

The folded fiber geometry shown in FIG. 13 therefore represents asignificant improvement over more conventional stacked ceramictransducers. Stacked ceramic transducers have traditionally been limitedin the number of stacked piezoelectric material layers that can becreated and/or in the thickness of the layers. For example, typicalmicrofabrication processes such as MEMs commonly permit only a few thinlayers of piezoelectric material to be deposited on a chip due to thegeneral difficulty of growing alternating layers of metals andpiezoelectric ceramics. Bulk piezoelectric transducers can bemechanically stacked, but the large film thickness inherent in suchdevices makes large driving voltages (e.g., from tens to hundreds ofvolts) a requirement.

The present disclosure enables formation of thin-layer piezoelectricmaterial stacks featuring tens or even hundreds of layers. The layerstacks can be assembled as preforms of macroscopic dimensions; as such,assembly of the selected fiber materials is generally relativelystraightforward. Thermal drawing, described previously, is then used toscale down the layers of piezoelectric material to thicknesses of a fewmicrons, while at the same time significantly increasing the surfacearea of the materials by drawing along the fiber axis. The drawingprocess thereby enables fabrication of fibers with internal multilayer,low-voltage, high sensitivity piezoelectric transducers.

Electrodes that contact the non-centrosymmetric material in the fibersdisclosed herein can be externally connected to various devices,including power supplies, electronic controllers/circuits, and/orelectronic processors. FIG. 14 shows an exemplary embodiment in which afiber 1400 is connected to an electronic control circuit 1418. In FIG.14, fiber 1400 includes a non-centrosymmetric material in region 1402, afirst conductor in region 1404 with a second conductor in region 1410contacting the first conductor in region 1404, the first conductor inregion 1406 with the second conductor in region 1414 contacting thefirst conductor in region 1406, and an insulator in region 1408.Electronic control circuit 1418 is connected to region 1410 via controlline 1420, and is connected to region 1414 via control line 1422.

In some embodiments, electronic control circuit 1418 is configured todirect electrical energy into fiber 1400 to generate an acousticwaveform in the fiber. That is, electronic control circuit is configuredto apply a varying electrical potential between regions 1410 and 1414.The application of a varying electrical potential generates an acousticwaveform in non-centrosymmetric material in region 1402 via the reversepiezoelectric effect. The generated acoustic waveform can, in someembodiments, propagate along fiber 1400 within region 1402. In general,if the electric waveform has a particular frequency, the generatedacoustic waveform has substantially the same frequency.

In certain embodiments, electronic control circuit 1418 is configured toreceive an electrical signal from fiber 1400. For example, if anexternal event or perturbation introduces an acoustic waveform into thenon-centrosymmetric material in region 1402 (e.g., an acoustic waveformthat impinges upon the non-centrosymmetric material), thenon-centrosymmetric material generates an electrical potentialdifference corresponding to the electrical waveform (e.g., atime-varying electric field) between regions 1404 and 1406 via thepiezoelectric effect, in response to the acoustic waveform. Electroniccontrol circuit 1418, by virtue of its connection to regions 1404 and1406 (e.g., through control line 1420 connected to regions 1410 and1404, and control line 1422 connected to regions 1414 and 1406), canreceive the electrical signal from fiber 1400 that corresponds to theelectric waveform as the acoustic waveform propagates in fiber 1400. Ingeneral, if the acoustic waveform has a particular frequency, theelectric waveform has substantially the same frequency.

Based on the measured electrical signal, electronic control circuit 1418can determine a property associated with the acoustic waveform in region1402. In some embodiments, electronic control circuit 1418 can determineone or more of an amplitude of the acoustic waveform, a frequency of theacoustic waveform, and a propagation velocity of the acoustic waveform.Based on these waveform properties, electronic control circuit 1418 canalso determine, for example, characteristics of other devices connectedto fiber 1400 (e.g., devices that introduce the acoustic waveform intofiber 1400).

Although in FIG. 14 electrical connections to the electrodes of a fiberare made at the end of the fiber, such electrical connections can moregenerally be made at any position along the fiber where the electrodesare accessible. In particular, some fibers include regions along thelength of the fiber where conductors internal to the fiber are exposedfor purposes of electrically contacting the conductors. By virtue oftheir construction, certain conductors in such fibers can extend to anouter surface of the fiber.

FIG. 15A shows a perspective view of an embodiment of a fiber 1500 thatincludes recesses that expose electrodes internal to the fiber. Fiber1500 includes a non-centrosymmetric material in region 1502, a firstconductor in regions 1504 and 1506, and an insulator in region 1508.Fiber 1500 also includes a second conductor in regions 1510, 1512, 1514,and 1516. The second conductor in regions 1510 and 1512 contacts thefirst conductor in region 1504; regions 1510 and 1512 can beelectrically contacted at an end of fiber 1500, for example. The secondconductor in regions 1514 and 1516 contacts the first conductor inregion 1506; regions 1514 and 1516 can be electrically contacted at anend of fiber 1500. However, the insulator in region 1508 includesrecesses 1518 formed in the insulator such that region 1516 extends toan outer surface of fiber 1500 through recesses 1518. Also present butnot visible in FIG. 15A are corresponding recesses in the insulator inregion 1508 that allow region 1514 to extend to the outer surface offiber 1500 on the other side of the fiber.

FIG. 15B shows a cross-sectional view of the fiber shown in FIG. 15Athrough section line A-A. By virtue of recesses 1518, an external devicesuch as electronic control circuit 1518 can directly contact regions1514 and/or 1516 at locations along the length of fiber 1500 in additionto, or as an alternative to, contacting these regions at the end(s) offiber 1500. Recesses 1518 thereby permit more flexible connections tointernal elements of fiber 1500 for purposes of constructing functionaldevices. In some embodiments, lateral connections to internal fiberelements may be more structurally sound than connections formed at theends of such fibers. Moreover, lateral connections may enable a greaternumber of circuits, processors, and other devices to be connected tosuch fibers.

Although recesses 1518 permit external connections only to regions 1514and 1516 in fiber 1500, more generally recesses and other suchstructural features (e.g., grooves, channels, depressions, openings) canbe used to permit access and connections to other internal elementswithin the fibers disclosed herein, including any of the regions of thefirst or second conductors present in the fibers. Further, although tworectangular recesses are shown in FIG. 15A on each side of fiber 1500,more generally a fiber can include any number and/or pattern of recessesformed in the insulator of region 1508. The recesses can have any shapeconvenient for purposes of facilitating contact between external devicesand the exposed fiber elements, including rectangular, square, circular,elliptical, and other regular or irregular shapes.

In some embodiments, a spacer material can be positioned between thenon-centrosymmetric material in the active region of a fiber and thefirst conductor in one or more of the regions that are adjacent to theactive region. Spacer materials can be used, for example, to improveadhesion between conductors and non-centrosymmetric materials, whilestill maintaining good electrical contact between the materials.Exemplary spacer materials that can be used in the fibers disclosedherein include any one or more of the thermoplastic materials disclosedherein.

Applications

A wide variety of applications are enabled by the fibers disclosedherein. The fibers enable detection and emission (transduction) ofacoustic waveforms at frequencies that range from tens of Hz to tens ofMHz. Thus, for example, the fibers can be used as pressure sensorsoperating at frequencies of a few hundred Hz. At higher frequencies,arrays of fibers can be used as detection elements (e.g., in ultrasoundimaging systems operating at tens of MHz). Due to the flexible andreproducible nature of the fiber drawing process, preforms can bereadily constructed that incorporate desired functional elements of thefibers, and then the preforms can be drawn to yield the fibers. As such,specialized fibers can be readily engineered for specific applications.

In some embodiments, the fibers disclosed herein can be integrated withother functional elements. For example, as the conducting materials inthe fibers disclosed herein correspond to electrical transmissionelements, any of the conducting materials can be electrically connectedto electronic elements such as power sources, transistors, amplifiers,and electrical gates.

In certain embodiments, some of the materials in the fibers can be usedto guide optical waveforms. For example, the insulating material can beconfigured to guide optical waveforms introduced into the fiber, eitherin a central portion of the fiber (e.g., in a hollow portion of thefiber enclosed by region 104 in FIG. 1A), or in an outer portion of thefiber (e.g., in a portion of the insulator in region 108 in FIG. 1A).Further, in some embodiments, the fibers can include additional regionsthat include materials having different optical properties. For example,the fibers disclosed herein can include one or more additional regionsformed of a material configured to guide optical waveforms along alength of the fiber. The one or more additional regions can extend alongthe fiber (e.g., along the common length of the fiber) to form opticaltransmission elements. As discussed previously, each of the additionalregions can be formed of one or more materials with a viscosity of lessthan 10⁷ Poise and which maintain structural integrity and chemicalcomposition at the common fiber draw temperature, to ensure thatperforms can be reproducibly drawn into high quality fibers.

In certain embodiments, one or more optoelectronic components or devicescan be adjacent to and/or contacted with fiber elements including one ormore of the conductors and/or the insulator. Exemplary optoelectroniccomponents can include photodetectors, light sources such as lasers, andmodulators such as acousto-optic modulators.

In some embodiments, the fibers disclosed herein can also include otherfunctional components integrated into the fiber structure. For example,the fibers can include integrated optical components such as reflectors,Fabry-Perot cavities, photonic bandgap devices, and filters. A varietyof different on-fiber applications can be enabled by such integration,including acousto-optic modulation.

To implement such integrated optical devices, the fibers can include oneor more additional regions formed of materials with different opticalproperties, the additional regions extending along the length of thefibers (e.g., the common length of the fibers). The materials present inthe additional regions are selected such that at the common fiber drawtemperature, they have a viscosity of less than 10⁷ Poise, and maintainstructural integrity and chemical composition.

Further, in certain embodiments, the fibers disclosed herein can includeadditional regions that include materials with different electricalproperties. The additional regions extend along the length of the fibers(e.g., along the common length of the fibers). As above, the materialshave a viscosity of less than 10⁷ Poise, and maintain structuralintegrity and chemical composition at the common fiber draw temperature.The materials in the additional regions can be positioned to implement avariety of different electronic devices within the fibers. For example,one or more of the additional regions can function as an electricaltransmission line within the fiber. In some embodiments, the additionalregions can implement more complex electronic devices such as signaldetectors, transistors, amplifiers, resistors, gates, junctions, and/orlogic devices.

Arrays of the acoustic fibers disclosed herein can be used forapplications such as acoustic imaging; the fiber arrays can cover areastoo large for conventional chip-based piezoelectric devices tosuccessfully image. The fiber arrays can be assembled into flexiblefabrics that permit real-time or near real-time imaging for applicationssuch as emergency-care medicine and non-destructive mechanical testingof materials. For example, once woven into fabrics, the fibers arecapable of acting as a communications transceiver. With smallcross-sectional dimensions (e.g., on the order of tens to hundreds ofmicrons) and long lengths (tens of meters or more), the fibers can beused to perform accurate pressure and/or flow measurements in smallblood vessels (e.g., intercranial vessels), and in vivo endovascularimaging and microscopy within acoustically-opaque organs, for example.

In certain embodiments, the fibers disclosed herein can be used for avariety of remote sensing applications. The sensitivity of the fibers tomechanical perturbations such as stress and strain, and their lowprofile, make the fibers ideal functional elements that can be used toconstruct sparse sensor meshes for investigating large-areadistributions of pressure and velocity fields in a variety of fluid flowapplications (e.g., oceanic current monitoring).

In some embodiments, the fibers disclosed herein can be used forcontrolled release and/or delivery of substances. For example, as shownin the cross-sectional view of FIG. 16, a fiber 1600 can include areservoir structure 1610 positioned at an end of the fiber. A valve 1612traps molecules 1605 of a substance within reservoir structure 1610.Fiber 1600 also includes a non-centrosymmetric material in region 1602,a first conductor in regions 1604 and 1606, and an insulator in region1608. Walls of reservoir 1610 are formed by the insulator in region1608, the first conductor in regions 1604 and 1606, and thenon-centrosymmetric material in region 1602. In some embodiments, onlysome of the materials of fiber 1600 may be used to form the walls ofreservoir 1610. For example, reservoir 1610 may be positioned entirelywithin region 1602, or entirely within region 1608.

The plurality of molecules 1605 within reservoir 1610 can becontrollably released. For example, the molecules can correspond toscents (e.g., perfumes) intended for release into the environmentsurrounding fiber 1600. Alternatively, the molecules can correspond toactive chemical agents such as drugs that are intended for release whenfiber 1600 is positioned at a particular site within a patient's body.

When release of the molecules contained within reservoir 1610 isdesired, an acoustic wave is generated in fiber 1600, e.g., by applyinga time-varying electric field between regions 1604 and 1606 in portion1614 of fiber 1600. A propagating acoustic waveform 1616 is generated bythe non-centrosymmetric material in region 1602 as a result. Acousticwaveform 1616 propagates along region 1602 until it reaches reservoir1610. The presence of acoustic waveform in the portion of region 1602that forms a wall of reservoir 1610 alternately enlarges and reduces anopening in valve 1612, permitting some of the molecules trapped withinreservoir 1610 to leave the reservoir through the valve. Further, aportion of the acoustic energy of acoustic waveform 1616 is transferredto the trapped molecules 1605, increasing their kinetic energy and theirdiffusion rate through the opening in valve 1612.

As a result, trapped molecules can be controllably released fromreservoir 1610 at desired locations and times. Moreover, because fiber1600 efficiently propagates acoustic waveform 1616, the delivery end offiber 1600 (e.g., the end in which reservoir 1610 is positioned) can beinserted into a patient, while the opposite end of the fiber (e.g.,where acoustic waveform 1616 is introduced) remains outside of thepatient.

In general, valve 1612 can be formed of any material that permitsmolecules 1605 to pass through when acoustic waveform 1616 creates amechanical disturbance in the valve. In some embodiments, for example,valve 1612 can be formed of a material with a permeability that can bevaried by introducing an acoustic waveform into the material.

Molecules 1605 can also have one or more properties that can assist intheir selective release at a target site. For example, in someembodiments, molecules 1605 include particles with a particle size thatcan change when excited by an acoustic waveform. By coupling an acousticwaveform into reservoir 1610, the sizes of the particles can be reduced,facilitating exit from reservoir 1610 through valve 1612.

In certain embodiments, molecules 1605 have a viscosity and/or adiffusivity that can change when excited by an acoustic waveform. Bycoupling an acoustic waveform into reservoir 1610, the viscosity ofmolecules 1605 can be reduced and/or the diffusivity of molecules 1605can be increased, facilitating exit of the molecules from reservoir1610.

In FIG. 16, valve 1612 is positioned so that molecules 1605—when leavingreservoir 1612—enter a region outside fiber 1600. In some embodiments,valve 1612 can be positioned so that molecules 1605, when the leavereservoir 1610 through valve 1612, enter a hollow portion of fiber 1600,e.g., the hollow central region of fiber 1600. Reservoir 1610 can alsoinclude more than one valve; certain valves can lead to regions externalto fiber 1600, while others can lead to internal regions of fiber 1600.

Further, in FIG. 16, reservoir 1610 is formed within one or more ofregions 1602, 1604, 1606, and 1608. In certain embodiments, however, thehollow central region of fiber 1600 can function as a reservoir inaddition to, or as an alternative to, a reservoir positioned as in FIG.16. Molecules 1605 can be trapped within the hollow region of fiber 1600by a valve 1612. Acoustic excitation of the valve and/or the trappedmolecules can lead to dispersal of some of the molecules into a regionoutside fiber 1600.

Although FIG. 16 shows a fiber 1600 with a single reservoir 1610 withtrapped molecules or particles, more generally the fibers disclosedherein can include multiple reservoirs, each with an associated regionof non-centrosymmetric material. In some embodiments, for example,fibers can include two or more regions of non-centrosymmetric material,each having one or more associated reservoirs, and the one or morereservoirs associated with each region of non-centrosymmetric materialcan include different types of trapped molecules. By inserting suchfibers into patients, for example, controlled release of different typesof therapeutic agents at specific locations and times can be achieved.

EXAMPLES

The following examples are not intended to limit the scope of thedisclosure or the claims. Acoustic fibers were prepared according to themethods discussed above. Both cylindrical fibers (e.g., as shown in FIG.1A) and planar fibers (e.g., as shown in FIGS. 12A and 12B) werefabricated using P(VDF-TrFE) as the non-centrosymmetric material,conductive polycarbonate (e.g., polycarbonate doped with carbon) to formthe first conducting material, indium filaments to form the secondconducting material, and undoped polycarbonate as the insulatingmaterial. At the fiber draw temperature, the conductive polycarbonatematerial had a viscosity of 10⁵-10⁶ Poise. Upon cooling, the conductivepolycarbonate material had a resistivity of about 1˜10⁴ Ω·m within afrequency range from 0 Hz to several tens of MHz. As a result, theconductive polycarbonate material facilitated short-range (e.g.,hundreds of microns) charge transport on length scales that wereapproximately equivalent to the fiber cross-sectional dimensions.

The indium filaments and conductive polycarbonate (CPC) form electrodesextending along the full length of the fiber. Specifically, the CPCportion of each electrode provides large surface area contact with theP(VDF-TrFE), while the indium filaments function as a longitudinal “bus”that increases the conductivity of each electrode over the full lengthof the fiber. Scanning electron microscope images of exemplaryfabricated fibers are shown in FIGS. 17A and 17B. In addition, fiberswith folded internal geometries that included multiple layers ofP(VDF-TrFE) were also fabricated; a scanning electron microscope imageof an exemplary fiber is shown in FIG. 17C.

Embedding piezoelectric domains that can be poled in a fiber's crosssection allows fibers to be electrically actuated over broad frequencieson the one hand, and to function as sensitive broadband microphones onthe other. To-date, however, fibers for the most part have been made ofmaterials in the disordered glassy state precluding the symmetryrequirements for piezoelectricity. Moreover, the need to apply electricfields to the piezoelectric material implies inclusion of conductingelectrodes within the fiber cross-section, which presents a number ofsignificant processing challenges.

Using the methods disclosed herein, fiber materials are drawn frompreforms in a regime dominated by viscous forces allowing for internallow viscosity domains to be arranged in non-equilibrium cross sectionsconfined by viscous glassy boundary layers. Preforms can include amaterial such as poly(vinylidene fluoride) (PVDF), metal electrodes, andan insulating polymer. Thermally drawing such preforms would lead tofibers with a non-centrosymmetric material therein (the stress presentduring the fiber draw induces the non-polar a to the ferroelectric (3phase transition in the PVDF layer). However, a variety of differentmaterial processing challenges are manifest at different length scalesin such fibers. On the hundreds of microns length scale, the use ofcrystalline materials both for the piezoelectric layer and theelectrical conductors leads to the formation of multiple adjacent lowviscosity domains of high aspect ratio. These domains can undergo asignificant reduction in cross sectional dimensions during the fiberdraw, and are therefore susceptible to capillary breakup and mixing dueto flow instabilities. At the tens of microns length scale, layerthickness variations either in the lateral or in the longitudinaldirections can impede the formation of coercive fields needed forpoling. Further, on molecular length scales, even if capillary breakupwas kinetically averted and uniform sections of fibers at meter lengthswere drawn, such fibers might not exhibit piezoelectricity if the stressand strain conditions necessary to induce the thermodynamic phasetransition in PVDF are not sustained during the fiber draw process.

To address the various fiber material processing challenges, fibers wereconstructed using viscous and conductive carbon-loaded poly(carbonate)(CPC) layers that were used to confine the low viscosity crystallinepiezoelectric layer during the draw process. The CPC layers exhibitedhigh viscosity (10⁵˜10⁶ Pa·s) at the draw temperature, and adequateresistivity (1˜10⁴ Ω·m) over the frequency range from DC to tens of MHz,thus facilitating short range (hundreds of microns) charge transport onlength scales associated with the fiber cross section. Further, apiezoelectric polymer, poly(vinylidene fluoride-trifluoroethylene)copolymer (P(VDF-TrFE)), which assumes the ferroelectric β phasespontaneously upon solidification from the melt without application ofmechanical stress, was used as the non-centrosymmetric material in thedrawn fibers.

The constituent fiber materials were assembled as illustrated in FIG.20. A series of shells, including a 700 μm-thick layer of P(VDF-TrFE)(70:30 molar ratio, melt-pressed from pellets obtained from Solvay,Brussels, Belgium) and multiple 250 μm-thick layers of CPC wereassembled with indium filaments and a poly(carbonate) (PC) cladding. Theentire structure was consolidated at 210° C. to remove trapped gas andform high quality interfaces. The preform was then thermally drawn in afurnace at 230° C. into fibers more than 100 meters long. Drawingoccurred in a three-zone vertical tube furnace at a down-feed speed of 1mm/minute. Drawn fiber dimensions were monitored with laser micrometers.

The stability of the draw was monitored by a continuous in-linemeasurement of the external dimensions of the fiber by a lasermicrometer. The standard deviation was calculated over a 10 cm window inreal-time and maintained at a level below 1% by controlling the drawstress. Since uniformity of the external geometry of the fiber does notensure that of the internal structure, Scanning Electron Microscopy(SEM) was used to image cross-sections of the fiber and to evaluate theuniformity of the piezoelectric polymer layer thickness across itswidth. Among the drawn fibers, a standard deviation in the polymer layerthickness of 3% was measured.

To investigate the uniformity of structure along the length of thefiber, a one meter-long portion of the fiber was cut into 3 cm-longsegments, and the capacitance of each segment was measured. Fluctuationin the average thickness of the piezoelectric P(VDF-TrFE) layer shouldresult in a proportional fluctuation on the capacitance, and a standardcapacitance deviation among the sections of 4% was measured.

A scanning electron microscopy (SEM) image 2000 of the fiber crosssection in FIG. 20 shows the P(VDF-TrFE) layer (determined to be 40 μmthick from the image) sandwiched between CPC layers, with the shape andthe aspect ratio unchanged from those of the preform. FIG. 20 also showsa wide angle X-ray diffraction (XRD) scan 2010 of a P(VDF-TrFE)copolymer domain harvested from the drawn fiber, and a XRD scan 2020 ofthe preform. Both the drawn fiber and the preform exhibit identicaldiffraction peaks at 2θ=19.9, 35.2, and 40.7 degrees, which correspondto (200)/(110), (001) and (310)/(020), (111)/(201) and (400)/(220) peaksof the β-phase of P(VDF-TrFE), establishing that the drawn copolymer ineach fiber solidified in its β-phase. The crystallinity fraction ascalculated from XRD patterns was over 90%. The obtained fiber was thenpoled by applying through the internal fiber electrodes an electricfield in excess of 60 MV/m, a field strength greater than the reportedcoercive field strength for P(VDF-TrFE). Long lengths of fiber werereadily poled in this way.

To characterize the acoustic properties of the fabricated fibers, motionof the fiber surface in response to internal piezoelectric modulationwas first measured using a heterodyne optical vibrometer at kHzfrequencies (e.g., where fiber dimension is much smaller than theacoustic wavelength). The vibrometry apparatus included afrequency-swept (1530-1570 nm, at 80 nm/s) laser (Agilent Technologies,8164B) coupled to a fiber-optic Michelson interferometer and aFabry-Perot frequency reference. The two arms of the Michelsoninterferometer differed in length and generated a heterodyne beat tone.In one arm, light was focused onto the vibrating surface of the fiber,where the Doppler shift of the reflected light produced additionalfrequency-modulation sidebands on the beat tone. The Fabry-Perotreference provided real-time frequency calibration. With thisinformation, the frequency chirp in the swept-laser source wascompensated when processing the interferometry data.

As shown schematically in FIG. 21, fibers (e.g., fiber 2105) wereelectrically driven by a sine wave with a maximum amplitude of 10 V. Thevibrating fiber surface functioned as an oscillating reflector; thefrequency of incident light 2110 was Doppler-shifted, and thefrequency-shifted reflected light 2120 was measured by the opticalvibrometer. A series of frequency-modulation (FM) side bands spaced atthe modulation frequency ωD were observed in the spectrum 2130 of thereflected radiation. The amplitudes of the side bands were proportionalto the velocity amplitude of the vibrations. Spectrum 2140 shows aseries of side bands for the cylindrical fiber of FIG. 20 driven atseveral frequencies from 1.3 to 1.9 kHz. The measured side bandsestablish a macroscopic piezoelectric response from the embeddedferroelectric layers in the fibers. The side band amplitude modulationresponse was found at ˜−60 dB below the main beat tone around thesefrequencies.

Spectrum 2150 shows similar frequency-modulation side bands for arectangular fiber with an embedded planar piezoelectric layer,fabricated using a technique similar to the one described above (seebelow). The rectangular geometry coupled more efficiently to the opticalbeam leading to a marked improvement in the signal measured in the sidebands compared to the immediate background. It also lead to a 20 dBincrease in the side band amplitude with respect to the heterodynesub-carrier.

Direct acoustic measurements were then performed on the drawn fibers,using the fibers as both an acoustic sensor and an acoustic actuatorcentered at 1 MHz. Such a frequency range is typical in ultrasoundimaging applications. The apparatus used to perform the experiments isshown schematically in FIG. 18. As shown in FIG. 18, a computer wasconnected to both a function generator and an oscilloscope. When a fiberwas evaluated as a detector, the fiber was connected to the oscilloscope(e.g., as the “Acoustic Detector”) and a commercial piezoelectrictransducer was connected to the function generator (e.g., as the“Acoustic Transmitter”). The transducer and the fiber were coupledacross a water tank to match the acoustic impedance. The computer wasconfigured to generate an arbitrary electrical waveform via the functiongenerator. The commercial transducer, acting as the transmitter,received the electrical waveform and generated a corresponding acousticwaveform in the water tank. The fiber, acting as the detector, receivedthe propagating acoustic signal and generated a corresponding electricalsignal that was transmitted to the oscilloscope and the computer. Theelectrical waveform generated by the computer via the function generatorwas then compared to the waveform detected by the fiber.

In contrast, when the fiber was evaluated as a transmitter, thepositions of the fiber and the commercial piezoelectric transducer werereversed (e.g., the fiber was connected to the function generator andthe commercial transducer was connected to the oscilloscope) and theprocedure was repeated to again compare the computer-generated waveformto the detected waveform.

A more detailed schematic of the experimental apparatus is shown in FIG.22A. A water-immersion ultrasonic transducer 2220 (OlympusPanametrics-NDT, 1.0 MHz-centered) was coupled to a 30 mm long fiber2210 across a water tank 2230 to match the acoustic impedance. Fiber2210 was attached to the surface of water tank 2230 via immersion gel,with the piezoelectric layer facing towards transducer 2220. Thetransducer-to-fiber distance was approximately 97 mm, roughly 70acoustic wavelengths at 1 MHz. At MHz frequencies, capacitiveelectromagnetic coupling between the transducer circuit and the receivercharge amplifier 2240 can be significant even with careful shielding andgrounding. To separate the acoustic signals from the electromagneticinterference, pulsed excitation of fiber 2210 was used, and the receivedsignals were time-gated to exploit the 5 orders of magnitude differencein the propagation speed between acoustic and electromagnetic pulses.Temporal traces of the amplified voltages under a pulsed excitation weremeasured with a carrier frequency at 600 kHz and a 52 μs temporalenvelope at a 6.5 kHz repetition rate. The time delays of the receivedpulses were consistent with acoustic propagation in water at 1470±30m/s, as demonstrated by temporal scans 2250 in FIG. 22B showingmeasurements of the received pulses.

Frequency domain characterizations of flat rectangular piezoelectricfibers were performed with a fixed transducer-to-fiber distance, withpulsed fiber excitation, and with time-gated signal processing. Aspectrum 2260 showing the frequency response of a typical flat fiber isshown in FIG. 22C. The measured piezoelectric response of the fiber,both as a sensor and an actuator, essentially followed the intrinsicfrequency profile of the transducer. Although the frequency range waslimited here by the bandwidth of the transducer, polymeric piezoelectricelements are in principle broadband and the piezoelectric fibers couldhave operated at a far broader range of frequencies. For example,similar fibers were used to generate audible sound between 7 kHz and 15kHz with a driving voltage of 5V.

FIG. 19A shows a comparison between the computer-generated waveform(dotted line) and the measured acoustic signal (solid line) when thefiber acts as a detector. The frequency response of the fiber as adetector generally matched the frequency spectrum of the sourcetransducer (note that the ripples were due to acoustic resonances in thewater tank). FIG. 19B shows a comparison between the computer-generatedwaveform (dotted line) and the measured acoustic signal (solid line)when the fiber acts as a transmitter. The frequency response of thefiber transmitter included spectral features that were similar to thoseof the commercial transducer. As in FIG. 19A, ripples in the measuredacoustic response were due to resonances in the water tank.

To further, demonstrate the flexibility of the methods disclosed herein,a rectangular fiber (as discussed briefly above) was fabricated with aFabry-Perot (FP) optical cavity structure layered on an embeddedpiezoelectric element. FIG. 23 shows a schematic diagram of the fiberpreform 2310 being drawn into a fiber. To fabricate the fiber, amacroscopic preform 32 mm in width and 11 mm in thickness was assembled.The preform was 25 cm long and included a 1.4 mm-thick layer ofP(VDF-TrFE) contacted by CPC and indium electrodes, and sandwichedbetween protective PC plates. The preform was consolidated in a hotpress at 175° C. and subsequently drawn in a three-zone vertical tubefurnace with a top-zone temperature of 150° C., a middle-zonetemperature of 230° C., and a down-feed speed of 1 mm/minute. The fiberdimensions were monitored with laser-micrometers. A capstan speed of0.5-3 m/minute produced a fiber of width between about 2,000 and 600 μmand a length of several hundred meters.

A SEM image 2320 of an exemplary drawn rectangular fiber shows that thepiezoelectric drawn fiber was 800 μm wide and exhibited well-maintainedpreform-to-fiber dimensional ratio and adhesion of the structures.Reflectivity of the piezoelectric FP fiber was characterized with aFourier transform infrared (FTIR) microscope (Bruker Optics,Tensor/Hyperion 1000), revealing that reflectivity reached 90% at awavelength of about 1500 nm. The spectral dip associated with the FPresonant mode was identified at 1550 nm.

As described above, heterodyne interferometry was used to characterizethe fiber vibration produced by the embedded piezoelectric material. Thepiezoelectric FP fiber was electrically driven by a sine wave atfrequencies stepped from 1.5 through 8.5 kHz with a maximum driving waveamplitude of 10V. An optical probe beam was focused on the FP structureto take advantage of the enhanced reflection from the bulk structure.Spectrum 2330 shows the frequency dependence of the side band amplitudemeasured for the FP fiber; these results suggest signals enhanced ordamped by underlying acoustic resonances of the fiber sample.

The piezoelectric FP fibers were mechanically robust yet flexible, andwere capable of assembly into a fabric 2340 for large area coverage, asshown in FIG. 23. The color of the fabric resulted from reflection fromthe third order band of the Fabry-Perot optical structure embedded inthe fibers.

Other Embodiments

A number of embodiments have been described. Nevertheless, it will beunderstood that various modifications may be made without departing fromthe spirit and scope of the disclosure. Accordingly, other embodimentsare within the scope of the following claims.

What is claimed is:
 1. A fiber comprising a composite of at least threedifferent materials, said at least three different materials comprisinga conductor, an insulator, and a material that is in a microscopicnon-centrosymmetric crystalline phase, each material being disposed inone or more different cross-sectional regions of the fiber, with eachregion extending along a common length of the fiber, said conductorcomprising a conductive material disposed in direct contact with thenon-centrosymmetric crystalline phase material; and wherein at a commonfiber draw temperature said conductive material has a viscosity that ishigher than that of the material in a non-centrosymmetric crystallinephase.
 2. The fiber of claim 1, wherein at a common fiber drawtemperature, each of said at least three different materials has aviscosity that is less than about 10⁷ Poise.
 3. The fiber of claim 2,wherein at the common fiber draw temperature, each of said at leastthree different materials maintains structural integrity.
 4. The fiberof claim 2, wherein at the common fiber draw temperature, each of saidat least three different materials maintains chemical composition. 5.The fiber of claim 1, wherein the material that is in anon-centrosymmetric crystalline phase comprises a piezoelectricmaterial.
 6. The fiber of claim 5, wherein the piezoelectric materialcomprises a ferroelectric material.
 7. The fiber of claim 1, wherein theconductive material is a first conductor, and further comprising asecond conductor, the second conductor being disposed in contact withthe first conductor in one or more additional cross-sectional regions ofthe fiber that extend along the common length of the fiber.
 8. The fiberof claim 7, wherein the first conductor has a viscosity that is greaterthan about 10² Poise at the common fiber draw temperature and the secondconductor has a conductivity greater than that of the first conductor.9. The fiber of claim 7, wherein the second conductor electricallycontacts the first conductor along the common length of the fiber. 10.The fiber of claim 7, wherein: a first set of the cross-sectionalregions comprising the first and second conductors in electrical contactwith one another define a first electrode; a second set of thecross-sectional regions comprising the first and second conductors inelectrical contact with one another define a second electrode; one ofthe cross-sectional regions comprising the material that is in anon-centrosymmetric crystalline phase defines an active region; and thefirst and second electrodes are positioned on opposite sides of theactive region.
 11. The fiber of claim 10, wherein the first conductorhas a viscosity that is greater than about 10² Poise at the common fiberdraw temperature and the second conductor has a conductivity greaterthan that of the first conductor.
 12. The fiber of claim 10, wherein thefirst conductor in each of the electrodes contacts the material that isin a non-centrosymmetric crystalline phase in the active region and hasa viscosity greater than that of the second conductor.
 13. The fiber ofclaim 10, further comprising a spacer material positioned between thematerial that is in a non-centrosymmetric crystalline phase in theactive region and the first conductor in each of the electrodes.
 14. Thefiber of claim 10, wherein the material that is in a non-centrosymmetriccrystalline phase can sustain a field of 3 MV/m or more when anelectrical potential difference is applied between the first and secondelectrodes.
 15. The fiber of claim 10, wherein the material that is in anon-centrosymmetric crystalline phase in the active region causes anacoustic waveform to be emitted from the fiber when an electricalpotential difference is applied between the first and second electrodes.16. The fiber of claim 15, wherein a frequency of the acoustic waveformcorresponds to a frequency of the electrical potential difference. 17.The fiber of claim 10, wherein the material that is in anon-centrosymmetric crystalline phase in the active region generates anelectrical potential difference corresponding to an electrical waveformbetween the first and second electrodes when an acoustic waveformimpinges on the non-centrosymmetric material.
 18. The fiber of claim 17,wherein a frequency of the electrical waveform corresponds to afrequency of the acoustic waveform.
 19. The fiber of claim 7, whereinthe second conductor comprises one or more metals.
 20. The fiber ofclaim 19, wherein the second conductor comprises an alloy of two or moremetals.
 21. The fiber of claim 19, wherein the one or more metalscomprise at least one of bismuth, lead, tin, indium, cadmium, gallium,copper, aluminum, silver, gold, and zinc.
 22. The fiber of claim 1,wherein a cross-sectional shape of the conductor has a maximum lengthmeasured along an outer surface of the conductor and a thicknessmeasured in a direction orthogonal to the outer surface, and wherein aratio of the maximum length to the thickness is 3 or more.
 23. The fiberof claim 1, wherein the fiber has an outer perimeter that is one ofcircular, elliptical, rectangular, square, triangular, and polygonal inshape.
 24. The fiber of claim 1, wherein the fiber has a length of 10centimeters or more, and a maximum cross-sectional dimension of 2 mm orless.
 25. The fiber of claim 10, wherein the first conductor in each ofthe electrodes extends to an outer surface of the fiber along at least aportion of the common length of the fiber.
 26. The fiber of claim 1,wherein the fiber is drawn from a preform having a length that issmaller than a length of the fiber.
 27. The fiber of claim 1, whereinthe at least three different materials are disposed in an all-solidfiber cross section.
 28. The fiber of claim 1, further comprising: oneor more additional regions positioned within the cross-section of thefiber and comprising materials with different optical properties, theone or more additional regions forming an optical transmission elementextending along at least a portion of the common length of the fiber,wherein, at the common fiber draw temperature, each of the materialswithin the one or more additional regions has a viscosity that is lessthan about 10⁷ Poise and maintains structural integrity and chemicalcomposition.
 29. The fiber of claim 1, further comprising: one or moreadditional regions positioned within the cross-section of the fiber andcomprising materials with different optical properties, the one or moreadditional regions forming an optical device extending along at least aportion of the common length of the fiber, wherein, at the common fiberdraw temperature, each of the materials within the one or moreadditional regions has a viscosity that is less than about 10⁷ Poise andmaintains structural integrity and chemical composition.
 30. The fiberof claim 1, further comprising: one or more additional regionspositioned within the cross-section of the fiber and comprisingmaterials with different electrical properties, the one or moreadditional regions forming an electronic device extending along at leasta portion of the common length of the fiber, wherein, at the commonfiber draw temperature, each of the materials within the one or moreadditional regions has a viscosity that is less than about 10 ⁷ Poiseand maintains structural integrity and chemical composition.
 31. Thefiber of claim 1, wherein the insulator comprises a polymeric insulatingmaterial or a high temperature insulating material comprising at leastone of silica and silicate glass.
 32. The fiber of claim 31, wherein thepolymeric insulating material comprises a material having a high glasstransition temperature.
 33. The fiber of claim 31, wherein the polymericinsulating material comprises at least one of a polyimide material, apolysulfone material, a polycarbonate material, a polymethacrylatematerial, a polyester material, a polyacrylate material, a polyethersulfone material, a cyclic olefin material, and a fluorinated polymermaterial.
 34. The fiber of claim 31, wherein the polymeric insulatingmaterial comprises a thermoplastic material.
 35. The fiber of claim 1,wherein: the conductive material comprises a composite of a hostmaterial and conducting particulates; the host material comprises atleast one material selected from the group consisting of a polycarbonatematerial, a polyethylene material, an acrylonitrile-butadiene-styrenecopolymer material, an acetal copolymer material, a polypropylenematerial, a polyvinylidene fluoride material, and a polyetherimidematerial; and the conducting particulates comprise at least one materialselected from the group consisting of carbon particles, carbon fibers,carbon nanotubes, and stainless steel fibers.
 36. The fiber of claim 1,wherein the material that is in a non-centrosymmetric crystalline phasecomprises at least one of a poly(vinylidene fluoride) material, acopolymer material of vinylidene fluoride and trifluoroethylene, apolyvinyl chloride material, a copolymer material of vinyl acetate andvinylidene cyanide, a nylon polymer material, a nylon copolymermaterial, a polyacrylonitrile material, and a high temperature ceramicmaterial comprising at least one of lead zirconate titanate, quartz,barium titanate, and cadmium sulfide.
 37. The fiber of claim 1, whereinan electrical resistivity of the conductor is 10⁵ ohm·m or less.
 38. Thefiber of claim 1, wherein an electrical resistivity of the insulator is10⁸ ohm·m or more.
 39. The fiber of claim 1, wherein a dielectricstrength of the insulator is 10 MV/m or more.
 40. The fiber of claim 1,further comprising a reservoir positioned within the fiber.
 41. Thefiber of claim 40, wherein the material that is in a non-centrosymmetriccrystalline phase forms at least one wall of the reservoir.
 42. Thefiber of claim 40, wherein: the material that is in anon-centrosymmetric crystalline phase causes an acoustic waveform to beemitted from the fiber when an electrical potential difference isapplied between the first and second electrodes; and wherein theacoustic waveform is configured to cause a substance present within thereservoir to leave the reservoir through a valve positioned in a wall ofthe reservoir.
 43. The fiber of claim 42, wherein the valve ispositioned so that the substance leaving the reservoir emerges from anend of the fiber.
 44. The fiber of claim 42, wherein the valve ispositioned so that the substance leaving the reservoir emerges into ahollow region of the fiber.
 45. The fiber of claim 42, wherein the valvecomprises a material having a permeability for the substance that can bevaried by introducing an acoustic waveform into the material.
 46. Thefiber of claim 42, wherein the substance comprises at least one of: aplurality of particles having a particle size that can be varied byintroducing an acoustic waveform into the substance; a viscosity thatcan be varied by introducing an acoustic waveform into the substance;and a diffusivity that can be varied by introducing an acoustic waveforminto the substance.
 47. A drug delivery system, the system comprising:the fiber of claim 40; and an electrical source coupled to the fiber toselectively cause delivery of a drug material in the reservoir.
 48. Thefiber of claim 40, wherein the reservoir is positioned at leastpartially within a region comprising the insulator or at least partiallywithin a region comprising the first conductor.
 49. An acoustic wavedetector, the detector comprising: the fiber of claim 1; and anelectrical detector coupled to the fiber to detect an electrical signalproduced by the non-centrosymmetric material in response to the acousticwave.
 50. The detector of claim 49, further comprising a plurality offibers corresponding to the fiber of claim 1, wherein the plurality offibers are configured to form a fiber array.
 51. An acoustic wavegenerator, the generator comprising: the fiber of claim 1; and anelectrical source coupled to the fiber to selectively cause thenon-centrosymmetric material in the fiber to generate the acoustic wave.52. A method for producing a fiber comprising a composite of at leastthree different materials, the method comprising: assembling a fiberpreform comprising the at least three different materials, wherein theat least three different materials include a conductor, an insulator,and a material that is in a having microscopic non-centrosymmetriccrystalline phase, with said conductor comprising a conductive materialdisposed in contact with the non-centrosymmetric crystalline phasematerial; and drawing the preform into a fiber at a fiber drawtemperature at which said conductive material has a viscosity that ishigher than that of the non-centrosymmetric crystalline phase material,the fiber comprising each of the at least three different materialsdisposed in one or more different cross-sectional regions of the fiber.53. The method of claim 52, wherein at the fiber draw temperature, eachof the at least three different materials has a viscosity that is lessthan about 10⁷ Poise.
 54. The method of claim 52, wherein each of the atleast three different materials maintains structural integrity andchemical composition when the preform is drawn into the fiber.
 55. Themethod of claim 52, further comprising maintaining a drawing tension of5 grams or more per square millimeter of the preform cross-sectionalarea as the preform is drawn to form the fiber.
 56. The method of claim53, further comprising cooling the fiber after the preform is drawn,wherein the material that is in a non-centrosymmetric crystalline phasesolidifies in the non-centrosymmetric crystalline phase when the fiberis cooled.
 57. The method of claim 56, wherein applying the electricalpotential difference comprises applying a direct current potentialdifference of 10 V or more per micrometer of thickness of the materialthat is in a non-centrosymmetric crystalline phase.
 58. The method ofclaim 52, further comprising annealing the fiber for a period of onehour or more at a temperature of between 120° and 150° C.
 59. The methodof claim 52, wherein: a cross-sectional region that comprises thematerial that is in a non-centrosymmetric crystalline phase defines anactive region; the conductor is disposed in a first set ofcross-sectional regions that define a first electrode, and in a secondset of cross-sectional regions that define a second electrode, the firstand second electrodes being positioned on opposite sides of the activeregion; and the method further comprising applying an electricalpotential difference between the first and second electrodes to aligndomains within the material that is in a non-centrosymmetric crystallinephase.
 60. The method of claim 59, wherein the conductor is a firstconductor, and wherein the at least three different materials furthercomprise a second conductor disposed in one or more additionalcross-sectional regions of the fiber.
 61. The method of claim 60,wherein the second conductor is disposed in the first and second sets ofcross-sectional regions.
 62. The method of claim 61, wherein the secondconductor electrically contacts the first conductor in the first andsecond sets of cross-sectional regions.
 63. The method of claim 60,wherein the second conductor comprises one or more metals.
 64. Themethod of claim 63, wherein the one or more metals comprise at least oneof bismuth, lead, tin, indium, cadmium, gallium, copper, aluminum,silver, gold, and zinc.
 65. The method of claim 60, wherein the firstconductor has a viscosity that is greater than about 10² Poise at thefiber draw temperature, and the second conductor has a conductivitygreater than that of the first conductor.
 66. The method of claim 60,wherein the first conductor in each of the first and second sets ofcross-sectional regions contacts the material that is in anon-centrosymmetric crystalline phase in the active region, and has aviscosity greater than that of the second conductor.
 67. The method ofclaim 60, further comprising applying an oxidation inhibitor to thesecond conductor prior to drawing the preform into a fiber.
 68. Themethod of claim 60, further comprising applying a wetting promoter tothe second conductor prior to drawing the preform into a fiber.
 69. Themethod of claim 60, further comprising applying a flux to the secondconductor prior to drawing the preform into a fiber.
 70. The method ofclaim 52, wherein the relative positions and cross-sectional dimensionsof each of the at least three different materials are substantially thesame in the preform and the fiber.
 71. The method of claim 52, whereinthe fiber has a length that is at least about 100 times greater than alength of the preform.
 72. The method of claim 52, wherein the fiber hasa maximum cross-sectional dimension that is at least about 10 timessmaller than a maximum cross-sectional dimension of the preform.
 73. Themethod of claim 52, further comprising consolidating the preform priorto drawing the preform into a fiber.
 74. The method of claim 73, whereinconsolidating the preform comprises heating the preform under vacuum.75. The method of claim 52, wherein the fiber draw temperature isgreater than a melting temperature or a glass transition temperature ofthe conductor.
 76. The method of claim 52, wherein the fiber drawtemperature is between about 120° C. and about 500° C.
 77. The method ofclaim 52, wherein the preform has a length of less than 50 cm.
 78. Themethod of claim 52, wherein the preform is drawn to form a fiber havinga length of 1 meter or more.
 79. The method of claim 52, wherein theinsulator comprises a polymeric insulating material or a hightemperature insulating material comprising at least one of silica andsilicate glass.
 80. The method of claim 79, wherein the polymericinsulating material comprises at least one of a polyimide material, apolysulfone material, a polycarbonate material, a polymethacrylatematerial, a polyester material, a polyacrylate material, a polyethersulfone material, a cyclic olefin material, and a fluorinated polymermaterial.
 81. The method of claim 52, wherein: the conductive materialcomprises a composite of a host material and conducting particulates;the host material comprises at least one material selected from thegroup consisting of a polycarbonate material, a polyethylene material,an acrylonitrile-butadiene-styrene copolymer material, an acetalcopolymer material, a polypropylene material, a polyvinylidene fluoridematerial, and a polyetherimide material; and the conducting particulatescomprise at least one material selected from the group consisting ofcarbon particles, carbon fibers, carbon nanotubes, and stainless steelfibers.
 82. The method of claim 52, wherein the material that is in anon-centrosymmetric crystalline phase comprises at least one of apoly(vinylidene fluoride) material, a copolymer material of vinylidenefluoride and trifluoroethylene, a polyvinyl chloride material, acopolymer material of vinyl acetate and vinylidene cyanide, a nylonpolymer material, a nylon copolymer material, a polyacrylonitrilematerial, and a high temperature ceramic material comprising at leastone of lead zirconate titanate, quartz, barium titanate, and cadmiumsulfide.
 83. The method of claim 52, wherein assembling the fiberpreform comprises wrapping a layer of one of the materials aroundanother one of the materials.
 84. The method of claim 52, whereinassembling the fiber preform comprises evaporating a layer of one of thematerials or a semiconducting material onto another one of thematerials.
 85. The method of claim 52, wherein assembling the fiberpreform comprises at least one of: evaporating one or more layers of theconductor onto the material that is in a non-centrosymmetric crystallinephase; and wrapping one or more layers of the conductor around thematerial that is in a non-centrosymmetric crystalline phase.
 86. Themethod of claim 52, wherein assembling the fiber preform comprises atleast one of: evaporating a layer of the insulator onto the conductor;and wrapping a layer of the insulator around the conductor.
 87. Themethod of claim 52, wherein the conductor is a first conductor and theat least three materials further comprise a second conductor, andwherein assembling the fiber preform comprises applying the secondconductor to a portion of a surface of the first conductor.
 88. Themethod of claim 87, wherein applying the second conductor to a portionof the surface of the first conductor comprises applying a liquidpolymer solution to the portion of the surface of the first conductor,and positioning the second conductor to contact at least a portion ofthe liquid polymer solution on the first conductor.
 89. The method ofclaim 87, wherein the first and second conductors extend substantiallyalong the entire length of the drawn fiber.
 90. The method of claim 52,wherein assembling the fiber preform comprises positioning a sacrificialpreform element within the preform to define a hollow fiber region, andremoving the sacrificial preform element prior to drawing the preform.